T cells that have been assigned jobs are called immunocompetent . The full complement of different T cells is analogous to the tiles of a scrabble game . After birth the thymus decreases in size and significantly so at puberty. In these sites immune cells come in contact with various antigens. Upon contact with antigen T cells become activated and stimulated to divide and some form memory T cells .
Memory cells act much more quickly if the antigen is re-encountered and their response lasts longer.
These proteins bind to specific sites on target cells and are critical in regulating immune responses. The CD4 marker on T helper cells and macrophages is used by HIV to gain access to these cells. Effector T cells release chemical regulators called cytokines. Cytokines are proteins produced by immune cells affect the behavior of other cells. Interferon that is released by virus-infected cells diffuses to uninfected cells.