Cell-mediated immunity is a function of cytotoxic T cells and is most effective against microorganisms that live inside body cells. Viruses and some bacteria are examples of intracellular micro-organisms. Cell-mediated immunity is also involved with some allergic reactions, the control of tumors, and graft rejection.
Cell-mediated immunity is essential for fighting viral infections. When viruses infect cells, they direct the cells to make new viruses, which are then released to infect other cells. Thus, cells are turned into virus-manufacturing plants. While inside the cell, viruses have a safe haven from antibody-mediated immunity because antibodies cannot cross the cell membrane. Cell-mediated immunity fights viral infections by destroying virally infected cells. When viruses infect cells, some viral proteins are broken down and become pro-cessed antigens that are combined with MHC class I molecules and displayed on the surface of the infected cell (figure 14.15, step 1).
Cytotoxic T cells can distinguish between virally infected cells and noninfected cells because the T-cell receptor can bind to the MHC class I/viral antigen complex, which is not present on uninfected cells.
The T-cell receptor binding with the MHC class I/antigen com-plex is a signal for activating cytotoxic T cells (figure 14.15, step 2). Costimulation by other surface molecules, such as CD8, also occurs (figure 14.15, step 3). Helper T cells provide costimulation by releasing cytokines, such as interleukin-2, which stimulate activa-tion and cell division of cytotoxic T cells (figure 14.15, step 4).
Increasing the number of “daughter” helper T cells results in greater stimulation of cytotoxic T cells. In cell-mediated responses, helper T cells are activated and stimulated to divide in the same fashion as in antibody-mediated responses (see figure 14.10).
After cytotoxic T cells are activated by an antigen on the sur-face of a target cell, they undergo a series of divisions to produce additional cytotoxic T cells and memory T cells (figure 14.16). The cytotoxic T cells are responsible for the cell-mediated immune response, and the memory T cells provide a secondary response and long-lasting immunity in the same fashion as memory B cells.
Cytotoxic T cells have two main effects:
1. They release cytokines that activate additional components of the immune system. For example, some cytokines attract innate immune cells, especially macrophages. These cells are then responsible for phagocytosis of the antigen and the production of an inflammatory response. Cytokines also activate additional cytotoxic T cells, which increases the effectiveness of the cell-mediated response.
2. Cytotoxic T cells can come in contact with other cells and kill them. Virally infected cells have viral antigens, tumor cells have tumor antigens, and tissue transplants have foreign antigens that can stimulate cytotoxic T-cell activity. The cytotoxic T cells bind to the antigens on the surfaces of these cells and cause the cells to lyse.