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. Uninfected cells are stimulated to make antiviral proteins that prevent viral replication. Humans make three major forms of interferon. Gamma interferon also stimulates macrophages to carry out phagocytosis. These are s mall cytokines that control the migration and activation of other cells, especially phagocytic cells and lymphocytes.