Cd4

T-helper cell; recognizes antigen presented by MHC class II molecules

All nucleated cells present endogenous antigen (originated inside of the cell) in the groove of MHC class I molecules

Exogenous antigen

MHC class molecule

All nucleated cells present endogenous antigen (originated inside of the cell) in the groove of MHC class I molecules

Exogenous antigen

MHC class molecule

Antigen-presenting cells present exogenous antigen (originated outside of the cell) in the groove of MHC class II molecules

T-helper cell; recognizes antigen presented by MHC class II molecules

Antigen-presenting cells present exogenous antigen (originated outside of the cell) in the groove of MHC class II molecules

Figure 16.17 Antigen Recognition by T Cells (a) T-cytotoxic cell; (b) T-helper cell.

essence, the effector T-cytotoxic cells judge other cells. If one deems that an endogenous peptide presented by a host cell represents danger, it directs the death of that cell.

When an effector T-cytotoxic cell encounters a peptide: MHC class I complex it recognizes, it establishes intimate contact with the target cell and releases several pre-formed cytotoxins, molecules that are lethal to the cell, directly to that cell. The cyto-toxins include perforin, a molecule that forms pores in cell membranes, and a group of proteases. Recent evidence indicates that at the concentrations released in vivo, perforin simply allows the proteases to enter the target cell. Once inside that cell, the proteases facilitate reactions that induce apoptosis. The remains of the apop-totic cell are then quickly removed by macrophages; the effector T-cytotoxic cell survives and can go on to kill other targets. Killing the cell by inducing apoptosis rather than lysis minimizes the number of intracellular microbes that might spill into the surrounding area and infect other cells. Most microbes remain in cell remnants until they are ingested by macrophages.

Functions of Effector T-Helper (CD4) Cells

Effector T-helper cells are responsible for judging the significance of antigen presented by a specialized group of cells called professional antigen-presenting cells, or more simply, antigen-presenting cells (APCs). These cells have a type of MHC molecule called MHC class II, which presents antigen to T-helper cells (see figure 16.16b). The peptides presented by MHC class II molecules are from exogenous antigens, meaning they originated outside of the cell (see figure 16.17b). Examples

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