* Protective immunity against microbes is mediated by the early reactions of innate immunity and the later responses of adaptive immunity. Innate immune responses are stimulated by molecular structures shared by groups of microbes and by molecules expressed by damaged host cells. Adaptive immunity is specific for different microbial and nonmicrobial antigens and is increased by repeated exposures to antigen (immunologic memory).

* Humoral immunity is mediated by B lymphocytes and their secreted products, antibodies, and functions in defense against extracellular microbes. Cell-mediated immunity is mediated by T lymphocytes and their products, such as cytokines, and is important for defense against intracellular microbes.

* Immunity may be acquired by a response to antigen (active immunity) or conferred by transfer of antibodies or cells from an immunized individual (passive immunity).

* The immune system possesses several properties that are of fundamental importance for its normal functions. These include specificity for different antigens, a diverse repertoire capable of recognizing a wide variety of antigens, memory of antigen exposure, the capacity for rapid expansion of clones of antigen-specific lymphocytes in response to the antigen, specialized responses to different microbes, maintenance of homeostasis, and the ability to discriminate between foreign antigens and self antigens.

* Lymphocytes are the only cells capable of specifically recognizing antigens and are thus the principal cells of adaptive immunity. The two major subpopulations of lymphocytes are B cells and T cells, and they differ in their antigen receptors and functions. Specialized antigen-presenting cells capture microbial antigens and display these antigens for recognition by lymphocytes. The elimination of antigens often requires the participation of various effector cells.

* The adaptive immune response is initiated by the recognition of foreign antigens by specific lymphocytes. Lymphocytes respond by proliferating and by differentiating into effector cells, whose function is to eliminate the antigen, and into memory cells, which show enhanced responses on subsequent encounters with the antigen. The activation of lymphocytes requires antigen and additional signals that may be provided by microbes or by innate immune responses to microbes.

* CD4+ helper T lymphocytes help macrophages to eliminate ingested microbes and help B cells to produce antibodies. CD8+ CTLs kill cells harboring intracellular pathogens, thus eliminating reservoirs of infection. Antibodies, the products of B lymphocytes, neutralize the infectivity of microbes and promote the elimination of microbes by phagocytes and by activation of the complement system.

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