1. Executive Summary

Key Finding: Based on the published human evidence reviewed from January 2021 through July 2026, allogeneic mesenchymal stromal cells (MSCs) are immunologically recognized by recipients, but the literature does not demonstrate a recurring pattern of clinically significant destructive immune rejection following properly manufactured Wharton's jelly MSC therapy in orthopedic applications.

This publication evaluates a focused clinical question: from January 2021 through July 12, 2026, does the published human evidence demonstrate that immune responses to allogeneic mesenchymal stromal cells (MSCs) produce clinically meaningful harmful outcomes? The review emphasizes culture expanded Wharton's jelly and umbilical cord tissue derived MSCs while distinguishing these products from hematopoietic stem cell transplantation, cord blood transplantation, embryonic stem cells, induced pluripotent stem cells, and minimally characterized birth tissue products. The objective is to evaluate the strength of the available evidence rather than promote or discourage any particular commercial therapy.

The literature consistently demonstrates that allogeneic MSCs are immunologically recognized rather than immunologically invisible. Resting MSCs generally express HLA Class I molecules, inflammatory signals can increase HLA expression, donor specific anti HLA antibodies have been documented in some recipients, and transient infusion related reactions remain established clinical observations. However, across the reviewed human orthopedic literature, no recurring pattern was identified in which properly manufactured Wharton's jelly MSCs produced the destructive immune rejection typically associated with solid organ transplantation. Current evidence therefore supports immune recognition with generally limited clinical consequences while acknowledging important uncertainties surrounding repeat dosing, rare adverse events, immune sensitization, and long term surveillance.

At the same time, this review did not identify a recurring human clinical syndrome from 2021 through the evidence cutoff in which immune rejection of a properly manufactured Wharton’s jelly MSC product caused destructive joint injury, permanent systemic organ damage, graft versus host disease, or death in an orthopedic recipient. Published orthopedic studies are small and cannot exclude rare events, but the available clinical record is materially different from the rejection biology seen in solid organ transplantation.

The strongest clinically relevant findings are summarized below.

• Immune recognition is real. The phrase immune privileged is too absolute for allogeneic MSCs.
• Transient fever is the most consistent treatment associated adverse event in large MSC safety meta analysis.
• Donor specific antibodies have been detected after some allogeneic MSC exposures, including studies involving bone repair and organ transplantation.
• Published studies often found antibodies without corresponding tissue injury, rejection, or loss of clinical benefit.
• Acute infusion reactions, hypersensitivity, pulmonary complications, and reactions to dimethyl sulfoxide are recognized safety concerns for intravenous cryopreserved products.
• Product contamination, poor manufacturing, incorrect administration, coagulation activation, and procedure related injury must not be mislabeled as immune rejection.
• Repeated exposure may matter more than a single administration, but the human evidence remains incomplete.
• Future organ transplant candidates may have a different risk calculation because anti HLA sensitization can complicate donor matching.

Bottom Line for Group Discussion

A scientifically defensible statement is: allogeneic MSCs can be recognized and cleared by the recipient’s immune system, and some recipients develop transient inflammatory reactions or donor specific anti HLA antibodies. However, the published human evidence from 2021 through July 2026 does not show a recurring pattern of classic destructive immune rejection caused by properly manufactured Wharton’s jelly MSCs in orthopedic treatment. Absence of a recurring published pattern is not proof that the risk is zero. The evidence base is limited by small trials, inconsistent immune monitoring, short follow up, product heterogeneity, and underreporting outside formal clinical studies.

2. Scope and Method

This is a structured narrative review rather than a registered systematic review. Priority was given to PubMed indexed human trials, systematic reviews, meta analyses, long term follow up studies, and official FDA documents published or made available from 2021 through July 12, 2026. Mechanistic reviews were included when necessary to interpret clinical observations. Preclinical findings were not treated as proof of human clinical harm.

The review separates five questions that are frequently blended together online:

1. Did the recipient’s immune system recognize the cells?
2. Were the donor cells cleared?
3. Did the recipient experience temporary inflammatory or infusion symptoms?
4. Did anti donor antibodies develop?
5. Did any of those responses cause a clinically important adverse health outcome?

A positive answer to the first four questions does not automatically establish the fifth.

3. Definitions That Prevent Misinterpretation

4. Why Allogeneic MSCs Are Not Completely Immune Invisible

MSCs are often described as having low immunogenicity. That statement can be reasonable when properly qualified. It becomes misleading when converted into the absolute claim that MSCs contain no HLA or cannot be recognized by the immune system.

Most nucleated human cells express HLA Class I molecules. Resting MSCs generally express HLA Class I and often have low or undetectable HLA Class II. Exposure to inflammatory signals, particularly interferon gamma, can increase HLA Class I and induce HLA Class II. Tissue source, donor biology, passage, culture media, cryopreservation, viability, cellular stress, dose, route, and recipient inflammation can all influence immunologic behavior.

Innate immune recognition may involve macrophages, monocytes, natural killer cells, complement, and pulmonary intravascular cells. Adaptive recognition may involve CD8 positive T cells, CD4 positive T cells, B cells, and anti HLA antibody production. The resulting clearance may be part of the mechanism through which MSCs influence immunity. Apoptotic MSCs can be engulfed by recipient phagocytes, a process that may contribute to immunomodulatory signaling rather than simply representing treatment failure.

1. Foundational 2021 review: Alloreactive Immune Response Associated to Human Mesenchymal Stromal Cells: https://pubmed.ncbi.nlm.nih.gov/34279481/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC8269175/
3. 2024 review on allo MSC immunogenicity and polarization: https://pubmed.ncbi.nlm.nih.gov/39405918/

5. What Counts as a Harmful Health Outcome

For this report, clinically meaningful harm includes death, permanent disability, organ failure, destructive inflammatory disease, severe hypersensitivity, sustained pulmonary compromise, clinically important thrombosis, hospitalization attributed to the product, loss of a transplanted organ, or another medically consequential outcome. Temporary fatigue or low grade fever is still an adverse event, but it is not equivalent to destructive immune rejection.

Causation requires more than timing. A convincing immune mediated attribution would ideally include a compatible clinical syndrome, objective immune testing, exclusion of infection and contamination, product characterization, temporal coherence, and evidence connecting the immune finding to the tissue injury. Most MSC trials were not designed with that depth of immune investigation.

6. Evidence Review by Year

2021

Alloreactive Immune Response Associated to Human Mesenchymal Stromal Cells

This systematic review examined the generation of donor specific antibodies and the immunogenicity of allogeneic MSCs. It is central to the modern rejection of the simplistic immune privileged label. The review describes both innate and adaptive recognition and concludes that allogeneic MSCs can produce humoral responses. Its clinical importance is that donor recognition is biologically and clinically measurable. Its limitation is that antibody detection did not consistently translate into tissue injury or severe clinical disease.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/34279481/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC8269175/

The Safety of MSC Therapy Over the Past 15 Years

This 2021 meta analysis evaluated randomized clinical trials and remains one of the most useful broad safety sources. MSC administration was significantly associated with transient fever, with a pooled odds ratio of 3.65 and a 95 percent confidence interval from 2.05 to 6.49. The analysis did not show a broad increase in infection, thrombotic events, malignancy, or death attributable to MSC therapy. The main immune compatible safety signal was temporary fever rather than destructive rejection.

Interpretive limitation: the analysis combined different tissue sources, manufacturing systems, diseases, doses, and routes. It supports an overall safety pattern but cannot certify a specific commercial product.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/34663461/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC8522073/

Percutaneous Allogeneic Bone Forming Cell Therapy and Anti HLA Antibodies

A 2021 human bone repair study is one of the clearest examples of immune sensitization without an observed clinical rejection syndrome. The proportion of samples containing donor specific anti HLA antibodies increased from 8 of 22 before treatment to 13 of 22 at six months. The investigators reported no treatment mediated allogeneic immune reactions. Clinical improvement occurred despite the antibody finding.

This study is important because it demonstrates that antibody formation can be real while clinical harm remains absent or unproven. It also involved a bone forming cell product rather than Wharton’s jelly MSCs, so it should not be generalized without qualification.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/34174963/

Cross Matching of Allogeneic MSCs

Rowland and colleagues reported that immune compatibility influenced MSC persistence and function in experimental systems. The work supports the proposition that immune recognition may reduce efficacy even when it does not produce a severe adverse health outcome. This is mechanistic evidence, not a report of human clinical injury.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/33369287/

Wharton’s Jelly MSCs for Osteoporotic Vertebral Compression Fractures

This phase I and IIa study evaluated Wharton’s jelly derived MSCs with teriparatide in osteoporotic vertebral fractures. The study supported feasibility and did not identify a pattern of severe immune mediated rejection. The trial was small and not capable of excluding rare adverse events. It remains directly relevant to musculoskeletal use because it involved a perinatal MSC source and bone pathology.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/33326694/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC8028433/

Allogeneic MSCs in Chronic Active Antibody Mediated Kidney Transplant Rejection

A 2021 clinical trial used allogeneic MSCs in kidney transplant recipients with chronic active antibody mediated rejection. No serious adverse events were reported during six months after MSC treatment. These recipients were medically complex and immunosuppressed. The finding supports tolerability in that setting but cannot be transferred directly to non immunosuppressed orthopedic recipients.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/33628269/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC7953763/

2022

Alloreactivity of Allogeneic Mesenchymal Stromal Cells

This review further developed the position that MSCs should not be treated as universally immune privileged. It discusses cellular rejection, antibody responses, inflammatory licensing, and the potential influence of repeated administration. The publication strengthens biological plausibility but does not establish a recurring clinical disease caused by MSC rejection.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/35309297/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC8926542/

Adverse Events, Side Effects, and Complications in MSC Based Therapies

This broad 2022 review catalogued reported complications including fever, infection, thrombosis, embolic phenomena, fibrosis, abnormal tissue formation, and other safety concerns. It is useful because it prevents the discussion from being reduced to rejection alone. Serious complications can occur through mechanisms unrelated to HLA mismatch, including coagulation activation, tissue factor expression, cell aggregation, route, dose, contamination, and procedural injury.

1. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC9659480/

One Year Follow Up After Wharton’s Jelly MSC Treatment for Severe COVID 19

A small follow up report involving four recipients did not identify serious long term complications attributed to Wharton’s jelly MSC treatment. The sample was too small to estimate uncommon risk. It contributes to the absence of an obvious delayed rejection pattern but cannot prove long term safety.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/36373143/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC9635897/

Umbilical Cord Derived Cells in Knee Osteoarthritis

A 2022 systematic review of human umbilical cord derived cell therapy for knee osteoarthritis reported generally favorable clinical outcomes and did not identify a recurring severe immune mediated safety signal. The literature was heterogeneous and included limited patient numbers, variable products, and inconsistent adverse event reporting.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/35859650/

2023

Wharton’s Jelly Derived MSCs in Recent Onset Type 1 Diabetes

A randomized, placebo controlled phase I and II study evaluated an allogeneic Wharton’s jelly derived MSC product in recent onset type 1 diabetes. The authors concluded that treatment appeared safe and had potential to preserve beta cell function. The study did not report a destructive alloimmune syndrome. Its relevance is direct exposure of immunologically active patients to an allogeneic perinatal MSC product under trial conditions.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/37221247/

Kidney Transplant HLA Eplet Mismatch Substudy

A 2023 substudy of the Triton kidney transplant trial reported de novo donor specific HLA antibodies in 7 of 29 patients in the MSC group compared with 1 of 27 controls at six months. The antibodies were analyzed in relation to kidney donor HLA mismatch. This is an important clinical signal, but attribution is difficult because recipients simultaneously carried an HLA mismatched kidney graft. The finding cannot be interpreted as proof that MSC donor HLA caused the antibodies or that antibodies caused a harmful MSC reaction.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/36841928/

Pilot Trial of Umbilical Cord Tissue MSCs in Neonatal Hypoxic Ischemic Encephalopathy

A phase I trial treated neonates with one or two intravenous doses of allogeneic cord tissue MSCs. Safety was the primary objective. The study did not establish a severe immune rejection syndrome. Neonatal critical illness, small sample size, and intensive concurrent care limit generalization.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/37285522/

Wharton’s Jelly MSC Review

A 2023 concise review summarized Wharton’s jelly MSC biology, secretome, and clinical applications. It emphasizes immunomodulatory properties but does not support the claim that these cells contain no HLA. It did not identify a recurring pattern of destructive clinical rejection.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/37440921/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC10333601/

2024

Five Year Immune Follow Up in Kidney Transplant Recipients

A 2024 long term analysis examined donor specific antibodies in two small cohorts totaling 20 kidney transplant recipients who had received third party bone marrow derived MSCs. Three patients developed donor specific antibodies. The investigators evaluated whether repeated HLA mismatches between MSC donors and kidney donors predicted antibody formation. The available findings did not establish that MSC sensitization caused kidney rejection or graft loss.

The report described antibody detection without biopsy proven rejection in one patient and other rejection events without detectable donor specific antibodies. One graft failure occurred in a patient with BK virus nephropathy and no antibody evidence linking the outcome to the MSC donor. The authors considered the detected antibodies more likely related to the kidney donor than the MSC donor.

This is among the most relevant long term human studies because it directly examines antibodies and clinical consequences. It also illustrates why transplant studies are difficult to interpret: both the organ and the MSC product are allogeneic.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/39360092/
2. Full article: https://www.frontiersin.org/journals/genetics/articles/10.3389/fgene.2024.1436194/full

FDA Approval and Safety Review of RYONCIL

On December 18, 2024, the FDA approved RYONCIL, an allogeneic bone marrow derived MSC product, for steroid refractory acute graft versus host disease in pediatric patients two months of age and older. It is not a Wharton’s jelly product and is not an orthopedic therapy, but it is the most important United States regulatory assessment of an allogeneic MSC product.

The FDA identified acute infusion reaction as an important identified risk. Important potential risks included pulmonary complications, donor specific HLA antibodies, ectopic tissue formation, suspected transmission of infectious agents, hypersensitivity, adverse events associated with dimethyl sulfoxide, and new malignancy. The FDA stated that the risk of anti donor or anti HLA antibodies and their effect on clinical outcome was unknown.

The agency required enhanced pharmacovigilance for anti donor antibody events and ectopic tissue formation. The available data did not show a safety signal requiring a formal Risk Evaluation and Mitigation Strategy or a safety related postmarketing study. This is a balanced regulatory conclusion: antibody risk is plausible and important enough to monitor, but a proven pattern of antibody mediated clinical injury had not been established.

1. FDA summary basis for approval: https://www.fda.gov/media/185192/download
2. FDA approval announcement: https://www.fda.gov/news-events/press-announcements/fda-approves-ryoncil-first-mesenchymal-stromal-cell-therapy
3. FDA prescribing information: https://www.fda.gov/media/184892/download

Wharton’s Jelly and Knee Osteoarthritis Review

A 2024 review concluded that published evidence suggested Wharton’s jelly and associated MSCs appeared safe and potentially effective for knee osteoarthritis. The authors also emphasized controversy regarding efficacy and the need for adequately powered randomized trials. The review did not identify destructive immune rejection as a recurring complication.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/38061765/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC10993459/

Meta Analysis of MSC Transplantation for Knee Osteoarthritis

A 2024 meta analysis included 16 studies and 875 patients. The authors concluded that MSC transplantation was generally safe and effective for pain and function, while imaging outcomes were less persuasive. This analysis included different cell sources and did not provide definitive immune surveillance. It supports the absence of a common severe safety pattern but cannot rule out rare immune reactions.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/37991925/

2025

Systematic Review of Wharton’s Jelly MSCs for Knee Osteoarthritis and Chondral Injury

A 2025 systematic review included six studies, 97 patients, and 134 treated knees, with follow up ranging from three months to four years. The review reported improvements in functional outcomes and no severe adverse effects. Imaging findings were mixed. The study population remains too small to exclude uncommon immune events, but no case of destructive immune rejection was identified.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/40137363/
2. Full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC11940407/

Allogeneic Umbilical Cord MSCs in Systemic Lupus Erythematosus

A 2025 phase I dose escalation study evaluated allogeneic umbilical cord derived MSCs in systemic lupus erythematosus. The trial provides contemporary safety evidence in a disease characterized by immune dysregulation. It did not establish a new syndrome of donor cell immune rejection. Small sample size and intensive clinical selection remain important limitations.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/39706212/

Human Bone Marrow Derived Allogeneic MSCs in Chronic Active Antibody Mediated Kidney Rejection

A 2025 clinical trial evaluated an allogeneic bone marrow derived MSC product in kidney recipients with chronic active antibody mediated rejection. This setting is useful for safety surveillance but difficult for causal interpretation because recipients already have an allogeneic organ, immune injury, and immunosuppressive treatment. The study did not provide evidence of a recurring MSC specific destructive rejection syndrome.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/40663031/

Immune Rejection of Human MSCs in a Preclinical Kidney Model

A 2025 publication reported immune rejection and increased circulating anti human antibodies after human MSC administration in an animal model. This supports biological plausibility but is not a human clinical report. Species mismatch can exaggerate immune recognition, so it must not be presented as evidence that human recipients routinely reject human allogeneic MSCs with clinical injury.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/40371959/

2026 Through July 12

Phase Ib Study of Wharton’s Jelly MSCs in Acute Graft Versus Host Disease

A phase Ib study published in 2026 evaluated Wharton’s jelly MSCs in acute graft versus host disease. The study focused on safety and preliminary activity in a highly complex patient population. It did not establish a destructive rejection syndrome caused by the MSC product. The indication, background immunosuppression, and disease severity prevent direct comparison with elective orthopedic treatment.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/41371057/
2. DOI: https://doi.org/10.1016/j.jcyt.2025.102012

Repeated Intravenous or Intrathecal Wharton’s Jelly MSCs

A 2026 open label trial compared repeated intravenous and intrathecal administration of GMP compliant Wharton’s jelly MSCs in a small neurologic population. The report contributes to contemporary repeated dose safety evidence but is too small to define uncommon immune risk. No recurring destructive alloimmune syndrome was established.

1. PubMed: https://pubmed.ncbi.nlm.nih.gov/40879603/

7. Evidence Matrix

8. Reported Clinical Reactions and Their Interpretation

9. Wharton’s Jelly MSCs and Orthopedic Use

The published orthopedic evidence from 2021 through the cutoff is reassuring but not definitive. Clinical studies and reviews involving Wharton’s jelly or umbilical cord derived MSCs generally report transient local discomfort, swelling, or nonspecific short term symptoms rather than severe immune disease. The 2025 systematic review of knee osteoarthritis and chondral injury found no severe adverse effects among 97 patients and 134 treated knees.

The absence of reported severe immune rejection must be interpreted within the size of the evidence base. A dataset of fewer than several hundred closely monitored recipients cannot reliably detect an event occurring once in several thousand treatments. Commercial treatment outside registered trials may also be underreported.

A claim that no immune response occurs is therefore inaccurate. A claim that severe immune rejection is common is also unsupported. The evidence supports low observed clinical immunogenicity under studied conditions, with documented capacity for immune recognition and uncertain rare event risk.

10. Why the 2019 Contamination Cases Are Different

The widely discussed United States outbreak associated with unapproved umbilical cord blood derived products involved bacterial contamination and serious infections. That event falls outside the requested 2021 through 2026 period and involved products that were not necessarily culture expanded Wharton’s jelly MSC preparations. It is included here only to prevent category confusion.

Sepsis from a contaminated product is a manufacturing and infection control failure. It is not evidence that the recipient’s immune system rejected viable donor MSCs. The immune system responds vigorously during infection, but the initiating cause is microbial contamination rather than HLA mismatch.

The practical lesson is that product sterility, endotoxin, mycoplasma testing, donor screening, chain of custody, viability, identity, and release criteria may be more immediately important to patient safety than theoretical immune privilege.

11. Why Lowering Excessive Inflammation Before Treatment May Matter

MSCs respond to the inflammatory environment they enter. Controlled inflammatory licensing can increase immunomodulatory functions, but intense or persistent inflammatory signaling can also increase HLA expression, promote apoptosis, alter secretory behavior, activate coagulation pathways, and accelerate immune clearance. This does not mean inflammation should be eliminated. Normal healing requires inflammatory signaling.

The practical objective is not to suppress every immune signal. It is to avoid unnecessary uncontrolled inflammation and address modifiable risks when medically appropriate. Examples include active infection, poorly controlled metabolic disease, smoking, severe sleep deprivation, and other conditions that may influence the recipient environment. No clinical trial has proven that a particular supplement protocol or short term anti inflammatory regimen prevents MSC alloimmunization.

12. Clinical and Manufacturing Variables That Change Risk

• Cell identity and purity. A product labeled stem cells may contain a different or mixed cell population.
• Sterility, endotoxin, and mycoplasma testing.
• Donor infectious disease and medical screening.
• HLA characteristics and degree of mismatch.
• Passage number and culture expansion conditions.
• Fresh versus cryopreserved product and residual DMSO.
• Viability and percentage of apoptotic or dead cells at administration.
• Dose, concentration, infusion rate, and number of administrations.
• Intravenous, intra arterial, intrathecal, intra articular, or local route.
• Tissue factor expression and coagulation compatibility.
• Recipient history of transplantation, transfusion, pregnancy, autoimmune disease, allergy, and preexisting anti HLA antibodies.
• Ability to recognize, document, and report adverse events.

13. Implications for Repeated Treatments

Repeated dosing creates a plausible opportunity for immune priming. The first exposure may generate memory T cells or antibodies that influence a later administration. Human studies have documented antibodies after allogeneic MSC exposure, but the frequency, persistence, dose relationship, and clinical effect remain inconsistent.

For a patient who may later require an organ or hematopoietic transplant, anti HLA sensitization deserves special consideration. Even an antibody that does not injure the MSC recipient could complicate future donor matching. Routine anti HLA testing is not standard in elective regenerative medicine, and evidence is insufficient to define who should be tested.

14. What the Literature Does Not Prove

• It does not prove that all Wharton’s jelly MSC products are equivalent.
• It does not prove that severe immune events can never occur.
• It does not establish the safety of unlicensed or poorly characterized commercial products.
• It does not show that every fever or episode of fatigue is an HLA mediated reaction.
• It does not establish that disappearance of donor cells means the therapy had no biological effect.
• It does not prove that donor specific antibodies are harmless in every recipient.
• It does not justify calling MSCs free of HLA.
• It does not support using organ transplant rejection as a direct model for MSC therapy.

15. Research Gaps Through July 2026

• Large prospective registries with standardized adverse event reporting.
• Routine baseline and follow up anti HLA antibody testing in selected trials.
• Linkage of antibody findings to donor HLA typing.
• Longer follow up after repeated allogeneic MSC administration.
• Head to head studies of Wharton’s jelly, bone marrow, adipose, placenta, and other sources.
• Standardized reporting of passage, viability, cryoprotectant, tissue factor, and potency.
• Better distinction between immune reactions, infusion reactions, infection, thrombosis, and disease progression.
• Orthopedic trials large enough to detect uncommon serious events.
• Independent postmarket surveillance of commercial regenerative medicine treatment.

16. Final Conclusion

The 2021 through July 2026 literature confirms that allogeneic MSCs can be recognized by the human immune system. Donor specific antibodies, cellular recognition, immune clearance, transient fever, and infusion reactions are real. The phrase immune privileged should therefore be replaced by a more qualified description such as relatively immune evasive or low immunogenicity under defined conditions.

Based on the currently published human evidence reviewed through July 2026, no recurring pattern of clinically significant destructive immune rejection following properly manufactured Wharton’s jelly MSC therapy for orthopedic indications was identified. The available orthopedic studies are small, but their safety findings are broadly consistent: serious immune mediated outcomes have not emerged as a common clinical problem.

The most accurate position lies between two extremes. It is incorrect to say there is no immune response to donor MSCs. It is also incorrect to imply that allogeneic MSCs commonly produce the type of destructive rejection seen after an unmatched organ transplant. Current evidence supports immune recognition with usually limited clinical consequences, while leaving meaningful uncertainty regarding repeated exposure, donor specific antibodies, rare adverse events, and poorly characterized commercial products.

This document is an educational review of published evidence and is not medical advice. Individual risk assessment requires a qualified physician who can evaluate the patient, the product, the route, the dose, and the manufacturing documentation.

17. Source List With Plain Text Links

1. 1. Sanabria de la Torre et al. Alloreactive Immune Response Associated to Human Mesenchymal Stromal Cells. 2021 https://pubmed.ncbi.nlm.nih.gov/34279481/
2. 2. Full text for Sanabria de la Torre et al. https://pmc.ncbi.nlm.nih.gov/articles/PMC8269175/
3. 3. Wang et al. The Safety of MSC Therapy Over the Past 15 Years. 2021 https://pubmed.ncbi.nlm.nih.gov/34663461/
4. 4. Full text for Wang et al. https://pmc.ncbi.nlm.nih.gov/articles/PMC8522073/
5. 5. Jayankura et al. Allogeneic bone forming cells and donor specific antibodies. 2021 https://pubmed.ncbi.nlm.nih.gov/34174963/
6. 6. Rowland et al. Cross matching of allogeneic MSCs. 2021 https://pubmed.ncbi.nlm.nih.gov/33369287/
7. 7. Shim et al. Wharton’s jelly MSCs for osteoporotic vertebral fractures. 2021 https://pubmed.ncbi.nlm.nih.gov/33326694/
8. 8. Full text for Shim et al. https://pmc.ncbi.nlm.nih.gov/articles/PMC8028433/
9. 9. Ban et al. Allogeneic MSCs in chronic active antibody mediated kidney transplant rejection. 2021 https://pubmed.ncbi.nlm.nih.gov/33628269/
10. 10. Full text for Ban et al. https://pmc.ncbi.nlm.nih.gov/articles/PMC7953763/
11. 11. Alloreactivity of allogeneic MSCs review. 2022 https://pubmed.ncbi.nlm.nih.gov/35309297/
12. 12. Full text for alloreactivity review. https://pmc.ncbi.nlm.nih.gov/articles/PMC8926542/
13. 13. Adverse Events, Side Effects and Complications in MSC Based Therapies. 2022 https://pmc.ncbi.nlm.nih.gov/articles/PMC9659480/
14. 14. One year follow up after Wharton’s jelly MSC treatment for severe COVID 19. 2022 https://pubmed.ncbi.nlm.nih.gov/36373143/
15. 15. Full text for COVID 19 follow up. https://pmc.ncbi.nlm.nih.gov/articles/PMC9635897/
16. 16. Umbilical cord derived stem cells for knee osteoarthritis review. 2022 https://pubmed.ncbi.nlm.nih.gov/35859650/
17. 17. Wharton’s jelly MSCs in recent onset type 1 diabetes. 2023 https://pubmed.ncbi.nlm.nih.gov/37221247/
18. 18. HLA DQ eplet mismatch and donor specific antibodies in kidney transplant MSC recipients. 2023 https://pubmed.ncbi.nlm.nih.gov/36841928/
19. 19. Umbilical cord tissue MSC trial in neonatal hypoxic ischemic encephalopathy. 2023 https://pubmed.ncbi.nlm.nih.gov/37285522/
20. 20. Wharton’s jelly MSC concise review. 2023 https://pubmed.ncbi.nlm.nih.gov/37440921/
21. 21. Full text for Wharton’s jelly MSC concise review. https://pmc.ncbi.nlm.nih.gov/articles/PMC10333601/
22. 22. Five year immune follow up in kidney transplant recipients. 2024 https://pubmed.ncbi.nlm.nih.gov/39360092/
23. 23. Full article for five year follow up. https://www.frontiersin.org/journals/genetics/articles/10.3389/fgene.2024.1436194/full
24. 24. FDA RYONCIL summary basis for approval. 2024 https://www.fda.gov/media/185192/download
25. 25. FDA RYONCIL approval announcement. 2024 https://www.fda.gov/news-events/press-announcements/fda-approves-ryoncil-first-mesenchymal-stromal-cell-therapy
26. 26. FDA RYONCIL prescribing information. 2024 https://www.fda.gov/media/184892/download
27. 27. Wharton’s jelly and osteoarthritis of the knee. 2024 https://pubmed.ncbi.nlm.nih.gov/38061765/
28. 28. Full text for Wharton’s jelly and knee osteoarthritis. https://pmc.ncbi.nlm.nih.gov/articles/PMC10993459/
29. 29. MSC transplantation for knee osteoarthritis meta analysis. 2024 https://pubmed.ncbi.nlm.nih.gov/37991925/
30. 30. Wharton’s jelly MSCs for knee osteoarthritis and chondral injury systematic review. 2025 https://pubmed.ncbi.nlm.nih.gov/40137363/
31. 31. Full text for 2025 knee systematic review. https://pmc.ncbi.nlm.nih.gov/articles/PMC11940407/
32. 32. Allogeneic umbilical cord MSCs in systemic lupus erythematosus. 2025 https://pubmed.ncbi.nlm.nih.gov/39706212/
33. 33. Allogeneic bone marrow MSC trial in chronic active antibody mediated kidney rejection. 2025 https://pubmed.ncbi.nlm.nih.gov/40663031/
34. 34. Preclinical immune rejection of human MSCs. 2025 https://pubmed.ncbi.nlm.nih.gov/40371959/
35. 35. Wharton’s jelly MSC phase Ib study in acute graft versus host disease. 2026 https://pubmed.ncbi.nlm.nih.gov/41371057/
36. 36. DOI for 2026 phase Ib study https://doi.org/10.1016/j.jcyt.2025.102012
37. 37. Repeated intravenous versus intrathecal Wharton’s jelly MSC trial. 2026 https://pubmed.ncbi.nlm.nih.gov/40879603/
38. 38. 2024 review on allo MSC immunogenicity and polarization https://pubmed.ncbi.nlm.nih.gov/39405918/

Publication and Copyright Notice

Copyright © 2026 Rewards Health. All rights reserved. This report was prepared by Will Lowe, Founder of RewardsHealth.com, as an independent educational review of the published literature. The report may be shared intact for educational discussion with attribution to Rewards Health. It may not be republished, altered, sold, or represented as clinical guidance without written permission from Rewards Health.

Suggested citation: Lowe W. Immune Responses and Clinical Outcomes Following Allogeneic Mesenchymal Stromal Cell Therapy: A Structured Review of Published Human Evidence From 2021 Through July 12, 2026. Rewards Health; 2026.

Medical information notice: This publication is educational and does not provide medical advice, diagnosis, or treatment recommendations. Clinical decisions should be made with an appropriately qualified physician who can evaluate the individual patient, product, route of administration, manufacturing documentation, and relevant medical history.

Limitations of the Evidence

Current conclusions should be interpreted in light of several important limitations. Most orthopedic studies involve relatively small sample sizes, heterogeneous manufacturing methods, short follow up periods, inconsistent immune monitoring, and variable adverse event reporting. These limitations reduce the ability to detect uncommon immune mediated events and reinforce the need for larger prospective studies.

Common Misconceptions

• Immune recognition does not necessarily equal clinically significant immune rejection.
• Limited persistence of administered MSCs does not necessarily indicate treatment failure.
• Pulmonary first pass cell trapping does not by itself demonstrate pulmonary injury.
• Detection of donor specific HLA antibodies does not automatically predict clinically meaningful harm.
• Manufacturing defects or contamination should not be confused with immune rejection.