Fig. 5.9 ). Here the antigens are processed and displayed complexed with MHC molecules on the cell surface, where the antigens are recognized by T cells. Similarly, soluble antigens are captured and concentrated in follicles in lymph nodes and the spleen, where they may be recognized by B cells via their antigen receptors.
At the same time as microbial antigens are recognized by T lymphocytes and B lymphocytes, the microbe http://ebooksmedicine.net
MACROPHAGE ACTIVATION, KILLING OF KILLING OF INGESTED MICROBES INFECTED CELLS
Fig. 5.9 Cell-mediated immunity. Dendritic cells (DCs) capture microbial antigens from epithelia and tissues and transport the antigens to lymph nodes. During this process, the DCs mature, and express high levels of MHC molecules and costimulators. Naïve T cells recognize MHC-associated peptide antigens displayed on DCs. The T cells are activated to proliferate and to differentiate into effector and memory cells, which migrate to sites of infection and serve various functions in cell-mediated immunity. CD4+ effector T cells of the TH1 subset recognize the antigens of microbes ingested by phagocytes, and activate the phagocytes to kill the microbes; other subsets of effector cells enhance leukocyte recruitment and stimulate different types of immune responses. CD8+ cytotoxic T lymphocytes (CTLs) kill infected cells harboring microbes in the cytoplasm. Some activated T cells remain in the lymphoid organs and help B cells to produce antibodies, and some T cells differentiate into long-lived memory cells (not shown). APC, Antigen-presenting cell.
activates innate immune cells expressing pattern recognition receptors. In the case of immunization with a protein antigen, as in a vaccine, a microbial mimic called an adjuvant that stimulates innate immune responses is given with the antigen. During the innate response, the microbe or adjuvant activates APCs to express molecules called costimulators and to secrete cytokines that stimulate the proliferation and differentiation of T lymphocytes. The principal costimulators for T cells are the B7 proteins (CD80 and CD86), which are expressed on APCs and are recognized by the CD28 receptor on naïve T cells. Antigen (“signal 1”) and costimulatory molecules produced during innate immune responses to microbes (“signal 2”) function cooperatively to activate antigen-specific lymphocytes. The requirement for microbe-triggered signal 2 ensures that the adaptive immune response is induced by microbes and not by harmless substances. In immune responses to tumors and transplants, “signal 2” may be provided by substances released from necrotic cells (the damage-associated molecular patterns mentioned  earlier).
The reactions and functions of T lymphocytes and B lymphocytes differ in important ways and are best considered separately.
Cell-Mediated Immunity: Activation of T Lymphocytes and Elimination of Intracellular Microbes