[Intracellular events in T lymphocyte's activation]

Adaptive immune responses are not initiated at the site where a pathogen first establishes a focus of infection. They occur in the organized peripheral lymphoid tissues, to which the pathogen or its products are transported, trapped, captured by specialized cells, called antigen-presenting cells (APC), which process and present the antigen to T lymphocytes. Activation of naive T cells requires two signals: the first signal is represented by the specific recognition of a foreign peptide fragment bound to a self MHC molecule, but this is not enough. The second signal, called co-stimulatory signal is represented by other molecules, expressed on the membrane or secreted by the APCs for which T cells express specific ligands. Binding of antigen to the T-cell receptor initiates a series of biochemical changes within the T cell, involving a large number of molecules.