Introduction

T cells are involved adaptive immunity:

Antigen presenting cells (APCs) internally process self and foreign proteins, cut them to short peptides (8-15 amino acids).

The peptides are presented on APC surface, attached to protein MHC.

T cell receptors (TCRs) recognize pathogen peptides by binding to them strongly.

T cell receoptors (TCRs) must be

Self-tolerant: to avoid auto-immune diseases

Diverse: to recognize an evolving landscape of pathogens

Specific: to lock on specific pathogens

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The ensemble of T cells is shaped in the thymus gland,

where a diverse repertoire of thymocites (expressing distinct TCRs) is culled by encounters with self-pMHC, via

Positive selection: TCR must bind sufficiently strongly to at least one self pMHC

(implicated in MHC restriction, and sensitivity).

Negative selection: TCR must not bind any self pMHC too strongly 

(deleting autoimmune TCR).

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