A mature T cell that leaves the thymus still does nothing until it is activated. Activation requires two signals delivered at the same time.
Signal 1 is TCR engagement: the TCR binds a specific peptide displayed on self MHC by an antigen-presenting cell. This recognition is the only step that gives the response its specificity. Signal 2 is costimulation, most commonly CD28 on the T cell binding CD80 or CD86 on the antigen-presenting cell. Costimulation does not add specificity; it confirms that the peptide is being presented by a cell that is genuinely responding to danger. Without signal 2, the T cell does not activate and instead becomes anergic, meaning it is refractory to later stimulation.
Once both signals are received, the TCR-associated kinases phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs) on the CD3 chains. This recruits and activates a cascade — ZAP-70, linker proteins, phospholipase C gamma, calcium release, and Ras-MAP kinase signaling — that ends in transcription factors such as NFAT, NF-kB, and AP-1. These factors drive interleukin-2 (IL-2) production, IL-2 receptor expression, and clonal proliferation.
A defect anywhere in this chain blocks the response. Loss of TCR or CD3 expression removes signal 1. Loss of a signaling kinase such as ZAP-70 interrupts the cascade after recognition has already occurred, so the cell binds antigen correctly but cannot respond. Loss of costimulation removes signal 2 and pushes the cell toward anergy. The clinical consequence is the same in each case — a T cell that is present in normal numbers but functionally silent — which is why these disorders are called signaling or functional defects rather than deficiencies of cell number.