• Antigen presentation stimulates T cells to become either "cytotoxic" CD8+ cells or "helper" CD4+ cells.

Figure 8.24 The stages of activation of CD4 T cellsNaive CD4 T cells first respond to their specific peptide:MHC class II complexes by making IL-2 and proliferating. These cells then differentiate into a cell type known as TH0, which has some of the effector functions characteristic of TH1 and TH2 cells. The TH0 cell has the potential to become either a TH1 cell or a TH2 cell.

• Function of T helper cells: Antigen presenting cells (APCs) present antigen on their Class II MHC molecules (MHC2). Helper T cells recognize these, with the help of their expression of CD4 co-receptor (CD4+). The activation of a resting helper T cell causes it to release cytokines and other stimulatory signals (green arrows) that stimulate the activity of macrophages, killer T cells and B cells, the latter producing antibodies

• Th1/Th2 Model for helper T cells. An antigen is ingested and processed by an APC. It presents fragments from it to T cells. The upper, Th0, is a T helper cell. The fragment is presented to it by MHC2. [1] IFN-γ, interferon γ; TGF-β, transforming growth factor β; mø, macrophage; IL-2, interleukin 2; IL-4, interleukin 4