Effector T-cytotoxic (CD8) cells recognize antigen presented by MHC class I molecules, and respond by inducing apoptosis in the target cell and secreting cytokines that stimulate surrounding cells to be more vigilant against intracellular invaders. Effector T-helper (CD4) cells of the subset Th1 recognize antigen presented by MHC class II molecules on macrophages, and respond by activating those macrophages and secreting various cytokines that help orchestrate the immune response. Effector T-helper (CD4) cells of the subset Th2 recognize antigen presented by MHC class II molecules on B cells, and respond by activating those B cells. T cells become activated to develop their effector functions when they recognize antigen presented by dendritic cells expressing co-stimulatory molecules.
■ Name three antigen-presenting cells.
■ Why would a person who has AIDS be more susceptible to the bacterium that causes tuberculosis?
16.8 Natural Killer (NK) Cells
Natural killer (NK) cells descend from lymphoid stem cells, but they lack the antigen-specific receptors that characterize B cells and T cells. Their activities, however, augment the adaptive immune responses. ■ lymphoid stem cells, p. 377
NK cells are important in the process of antibody-dependent cellular cytotoxicity (ADCC), which is a means of killing cells that have been bound by antibody (see figure 16.6). This provides the defense system with a mechanism to destroy parasitic organisms that are too large to be ingested by phagocytic cells. It also enables the killing of host cells that have foreign proteins inserted into their membrane, such as those that have been infected by certain types of viruses. NK cells recognize their target by means of Fc receptors for IgG antibodies on their surface; recall that Fc receptors bind the "red flag" portion of antibody molecules. These Fc receptors enable the NK cell to detect and attach to an antibody-coated cell. When multiple receptors on an NK cell bind the Fc regions, the NK cell delivers granules that contain perforin and proteases directly to the target cell. These compounds can kill foreign cells and induce apoptosis in "self" cells.
NK cells also recognize and destroy host cells that do not have MHC class I molecules on their surface. This is important because some viruses have evolved mechanisms to circumvent the action of T-cytotoxic cells by interfering with the process of
412 Chapter 16 The Adaptive Immune Response antigen presentation; cells infected with a virus that does this will be essentially bare of MHC class I molecules and thus cannot be a target of a T-cytotoxic cell. The NK cells can recognize the absence of MHC class I molecules and induce apoptosis in those cells. This can occur because NK cells are actually programmed to destroy "self" cells, but recognition of the MHC class I molecules suppresses that killing action. In the absence of MHC class I molecules, the action can proceed.
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