T Cell

T cells are lymphocytes that mature in the thymus and carry an antigen receptor assembled by somatic recombination, giving a repertoire far larger than the genome could encode directly. The receptor does not bind free antigen; it recognises a short peptide held in the groove of an MHC molecule, a constraint called MHC restriction. Development applies two filters: positive selection retains cells binding self-MHC weakly, negative selection deletes those binding self-peptide too strongly. The main effector lineages are CD4 helper cells reading class II and CD8 cytotoxic cells reading class I.

The clinical arithmetic is familiar from HIV medicine, where a CD4 count below 200 cells per microlitre defines advanced disease and predicts opportunistic infection. Engineering the same cells produced the CAR-T therapies, beginning with tisagenlecleucel in 2017, and blocking their inhibitory checkpoints produced the PD-1 and CTLA-4 antibodies. Memory subsets persist for decades, which is why a T-cell response to a vaccine is detectable long after antibody titres have fallen.

Numbers and function are separate measurements: a normal lymphocyte count says nothing about whether the repertoire contains cells specific for a given pathogen, which is why antigen-specific assays exist. CD4 help is also required for most high-affinity antibody responses, including the anti-drug antibodies raised against therapeutic peptides.

The misuse is the immune strength framing, in which a T-cell count from a routine panel is offered as a measure of competence that a supplement can raise. Counts fluctuate with cortisol, exercise, acute infection and time of day; they are diagnostic in specific deficiency states and near-uninformative in a healthy adult. Increased proliferation in vitro is a response to a mitogen, not evidence of protection.