Major Histocompatibility Complex (MHC)

The major histocompatibility complex is a gene cluster on chromosome 6 in humans, where its products are called human leukocyte antigens. Its job is to hold short peptide fragments where T-cell receptors can inspect them. Class I molecules are expressed on nearly all nucleated cells and present peptides of roughly eight to ten residues, drawn from proteins made inside the cell, to CD8 T cells. Class II molecules are restricted to professional antigen-presenting cells, present longer peptides taken up from outside the cell, and are read by CD4 T cells.

These are the most polymorphic loci in the human genome, with tens of thousands of alleles described, and the variation has direct clinical consequences. Screening for HLA-B*57:01 before abacavir is required because carriers face a high risk of severe hypersensitivity. HLA-B*15:02 is associated with carbamazepine-induced Stevens-Johnson syndrome in several Asian populations. Donor and recipient matching remains central to transplantation.

MHC restriction sets the boundary of what any peptide result means: a T-cell epitope is only an epitope in the context of an allele that binds it, so a sequence immunogenic in one person can be invisible to another. The same logic underlies the T-cell help that drives anti-drug antibody formation, and it is why in silico HLA binding prediction is now part of immunogenicity risk assessment.

The error is generalising across species and alleles. A peptide vaccine epitope validated in C57BL/6 mice was selected by mouse molecules with their own binding preferences and carries no guarantee of presentation by any human allele. Consumer HLA typing sold as a wellness measurement is a separate problem: outside drug-hypersensitivity and transplant settings, a haplotype report supports no dietary or supplement decision.