Human Serum Off-the-Clot Type AB
Human serum produced by natural whole blood clotting from blood type AB donors — the richest growth factor format of any human serum preparation. Platelet degranulation during natural clotting releases PDGF, EGF, TGF-β, IGF-1 and FGF directly into the serum. Universal blood type: free of anti-A and anti-B antibodies.
The Science of Off-the-Clot Serum — Why It Matters
What Happens During Natural Clotting
When whole blood is allowed to clot naturally without anticoagulants — the off-the-clot (OTC) process — the coagulation cascade proceeds from start to finish. Critically, platelets activate during this process and undergo full degranulation, releasing their entire alpha-granule and dense granule contents directly into the forming serum.
This platelet degranulation step is the defining feature of OTC serum. Platelets are dense stores of growth factors that are normally released only at sites of injury and tissue repair. During OTC preparation, their full growth factor payload is released into the serum:
| Growth Factor | Source | Key Function in Cell Culture |
|---|---|---|
| PDGF (AA, BB, AB) | Platelet alpha-granules | Mitogenic for fibroblasts, MSC, smooth muscle cells |
| EGF | Platelet alpha-granules | Proliferation of epithelial cells, fibroblasts, organoids |
| TGF-β1 | Platelet alpha-granules | MSC differentiation, immunomodulation |
| IGF-1 | Platelet alpha-granules | Survival and proliferation across multiple cell types |
| FGF | Platelet alpha-granules | Stemness maintenance, angiogenesis, fibroblast expansion |
| VEGF | Platelet alpha-granules | Endothelial cell support, angiogenesis models |
| Serotonin, ATP | Platelet dense granules | Metabolic signals, paracrine cell communication |
OTC vs. Plasma-Derived: The Critical Difference
Plasma-derived serum is prepared from plasma that has been separated from whole blood before clotting is induced. Because platelets are removed with the cellular fraction before the clotting step, platelet degranulation does not occur during the serum preparation process. The result is a product with significantly reduced concentrations of platelet-derived growth factors.
For cell expansion applications where proliferative support is the primary requirement, OTC serum consistently outperforms plasma-derived preparations. For applications requiring controlled, low-growth-factor conditions, plasma-derived serum may be preferred — but for MSC expansion, HSC culture, organoid initiation and T-cell expansion, OTC is the correct format.
OTC vs. Plasma-Derived Human Serum
| Parameter | OTC | Plasma-Derived |
|---|---|---|
| Collection method | Whole blood, natural clot | Plasma apheresis then induced clotting |
| Platelet degranulation | Yes — full | No — platelets removed pre-clotting |
| PDGF content | High | Low |
| EGF content | High | Lower |
| TGF-β content | High | Lower |
| Fibrinogen | Absent (consumed) | Absent (consumed) |
| Clotting factors | Absent (consumed) | Absent (consumed) |
| Scalability | More donors per batch | More scalable via apheresis |
| Cost | Higher | Lower |
| Best for | Cell expansion, MSC, HSC, organoids | Volume-sensitive, cost-driven applications |
Why Blood Type AB?
Blood group AB individuals express both A and B antigens on their red blood cells and — crucially — carry neither anti-A nor anti-B antibodies in their serum. This makes AB serum universally compatible for use with human cells of any blood type.
When serum from blood type A, B or O donors is used in cell culture, the anti-A or anti-B antibodies present can interact with blood group antigens expressed on human cell lines and primary cells, triggering complement-mediated effects or antibody-dependent interference. Type AB eliminates this risk entirely.
This is why Type AB is the standard human serum format for:
- Plasmodium falciparum culture (requires AB, no agglutination of infected erythrocytes)
- MSC and HSC expansion (human cells express A/B antigens)
- CAR-T manufacturing (consistent across patient blood types)
- PBMC culture for T-cell assays (avoids blood type-specific interference)
Applications
MSC Expansion — Bone Marrow, Adipose, Cord
Human AB serum (OTC) has been validated in multiple peer-reviewed studies for MSC expansion from bone marrow and adipose tissue. Pre-clinical data demonstrates performance equivalent to or superior to FBS for MSC proliferation while maintaining multipotency and immunophenotype (CD73+/CD90+/CD105+). Used at 5–10% in standard MSC expansion media and in microcarrier-based bioreactor systems.
Hematopoietic Stem Cell (HSC) Expansion
Human AB serum OTC supports ex vivo expansion of HSCs from peripheral blood, bone marrow and cord blood in haematopoietic stem cell expansion media. Used alongside cytokine cocktails (SCF, FLT3L, TPO, IL-3, IL-6) at 2–5% to provide growth factor support without xenogenic background.
CAR-T & T-Cell Expansion
Human Serum AB is the FDA-accepted human serum supplement for CAR-T and TIL manufacturing protocols. Used at 2–5% in TexMACS and similar T-cell expansion media. Provides growth support and xeno-free compliance without anti-AB antibody interference. OTC format preferred for maximum growth factor support during rapid T-cell proliferation phases.
Organoid Initiation & Expansion
Human AB OTC serum provides a growth factor-rich supplement for organoid initiation from primary tissue. Particularly useful where complete serum-free conditions cannot yet be established or when transitioning from serum-containing to defined media. OTC format ensures PDGF and EGF availability supporting early organoid formation.
Plasmodium falciparum Culture (Malaria Research)
The standard Trager & Jensen continuous P. falciparum culture protocol requires Type AB serum specifically — non-AB serum causes agglutination of infected erythrocytes. OTC format is preferred as complement is not detrimental at low concentrations and the growth factor content supports parasite-sustaining RBC integrity. Lot validation for parasite growth support is essential.
Primary Human Cell Culture — General
For human primary cell types where physiological serum composition matters — macrophages, dendritic cells, endothelial cells, chondrocytes, fibroblasts — Human AB OTC provides a human-matched, xeno-free matrix that more closely reflects the in vivo milieu than FBS. Used in translational in vitro models per Cela et al. (Clin Transl Sci, 2025).
Product Information
| Product type | Human Serum Off-the-Clot (OTC) — Type AB |
| Collection method | Off-the-clot — whole blood allowed to clot naturally without anticoagulants |
| Blood type | AB — pooled from AB donors only. No anti-A or anti-B antibodies. |
| Donor gender | Mixed (male and female). For male-only Type AB, see Human Serum Type AB Male. |
| Growth factor content | High — PDGF, EGF, TGF-β1, IGF-1, FGF, VEGF from platelet degranulation |
| Complement activity | Present in standard (non-HI). Heat inactivated available on request. |
| Filtration | 0.2 µm sterile filtered |
| Endotoxin | Tested per lot — LAL method, ≤10 EU/mL |
| Mycoplasma | Tested — negative |
| Viral screening | HBsAg, HCV, HIV-1/2 Ag+Ab, West Nile Virus NAT, Syphilis — negative per FDA requirements |
| Heat inactivation | Available on request (56°C, 30 min) — note: partially reduces growth factor activity |
| Origin | EU (Germany / Netherlands / France) · US — specify on order |
| Volumes | 10 mL, 50 mL, 100 mL, 500 mL · Bulk on request |
| Storage | −20°C · Batch reservation available · Max 3 freeze-thaw cycles |
| Documentation | CoA, CoO, SDS, Donor Screening Report, TSE/BSE Statement |
| Regulatory status | For research use only |
Human Serum OTC Type AB — Which Variant?
SeamlessBio offers four OTC/AB serum formats. This selection guide helps you choose the right one for your application.
Human Serum OTC Type AB
Mixed gender, blood type AB. Maximum growth factors, universal blood type. Standard choice for MSC, HSC, organoids, P. falciparum culture and general human cell expansion.
Human Serum OTC Type AB Male
Male donors only, blood type AB. Adds HLA alloantibody-free benefit on top of AB universality. Preferred for CAR-T, T-cell assays and ATMP manufacturing requiring maximum cleanliness.
View product →Human Serum OTC (Standard)
Mixed gender, mixed blood types. OTC growth factor profile without blood type restriction. For applications where blood type compatibility is not critical.
View product →Human Serum Type AB Male (Standard)
Standard (not OTC) human serum, male donors, blood type AB. Lower growth factor content than OTC — suitable where defined, consistent growth factor input is preferred over maximum content.
View product →Frequently Asked Questions
What does off-the-clot (OTC) mean?
Off-the-clot refers to serum produced from whole blood allowed to clot naturally, without anticoagulants. During natural clotting, platelets activate and degranulate — releasing their full growth factor payload (PDGF, EGF, TGF-β, IGF-1, FGF) into the serum. This makes OTC serum the growth-factor-richest human serum format available.
Why blood type AB?
AB donors carry neither anti-A nor anti-B antibodies. This makes AB serum universally compatible for human cell culture regardless of the blood type of the cells being cultured — essential for any application where anti-A or anti-B antibody interference would be detrimental, including P. falciparum culture, MSC expansion and CAR-T manufacturing.
Should I heat inactivate OTC Type AB serum?
For cell expansion (MSC, HSC, CAR-T, organoids) — do not heat inactivate. Growth factors (particularly TGF-β and EGF) are partially denatured at 56°C. Heat inactivation is appropriate for complement-sensitive immunological assays and hematopoietic cell culture where complement could cause unwanted lysis.
Is Human Serum OTC Type AB equivalent to FBS for MSC expansion?
Studies have shown Human AB serum (OTC) to be equivalent or superior to FBS for MSC expansion while providing xeno-free compliance. OTC Type AB was validated for MSC expansion from adipose tissue in spinner flasks and 500 mL bioreactors (PMC7184110). FBS superiority is not consistent in MSC applications — and the regulatory advantage of using human-derived serum for ATMP development is significant.
What concentration should I use?
MSC and HSC expansion: 5–10%. CAR-T and T-cell expansion: 2–5%. Organoid culture: 2–5% depending on growth factor requirements of the specific organoid system. P. falciparum: 10% standard (Trager & Jensen protocol). Start with 5% and titrate based on growth response and application-specific requirements.
Related Products & Guides
Request Human Serum OTC Type AB — Quote or Sample
EU and US origin available. Heat inactivated on request. Batch reservation for continuous culture programmes. Full documentation included.
For research use only.
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