Helper T cell receptors, activation, proliferation, differentiation & action

• Most cells which have CD4 on their surface become Helper T cells (TN cells). • The CD4 1 cells only recognize a foreign antigen when it is presented with an antigen presenting immune cell (APC) that includes MHC-II protein. • The Helper T cell antigen receptor must match the presented MHC-II antigen and bind together to begin activation. This is the first of two signals needed. • Costimulation, a second signal in addition to the attaching of the TCR and MCH-II antigen, is needed to activate a T cell. • Costimulation can be provided by plasma membrane molecules attached to the T cell and the APC cell, which adhere to one another, binding the cells tightly together. • Molecules, called "cytokines", released by the antigen presenting cell are costimulators. • Once the T cell is activated, it will proliferate into a clone of identical T cells, each expressing the T cell receptor of the original cell. • Most of the offspring cells will differentiate into effector T cells. • The receptors on these cells will be able to bind to the same antigen as the receptor on the T cell which was originally activated. • Some of the T cell offspring will not become effector cells. • These cells are memory T cells, which will respond rapidly upon future exposure to the same antigen. • After activation, helper T cells secrete molecules called cytokines. • The cytokine, Interleukin -2 (1L-2), is important in costimulating T cells in cell-mediated immunity. It also helps activate B and Natural Killer cells. • Interleukin-4 (IL-4), or B cell stimulating factor, is used as a costimulator for B cells. • Interleukin-5 (IL-5) is another cytokine involved in B cell costimulation. • Gamma-interferon (y-IFN), is important in stimulating nonspecific resistance, and is involved in both cell-mediated and antibody-mediated immunity.

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