The adaptive immune system uses three main strategies to combat most microbes.
• Secreted antibodies bind to extracellular microbes, block their ability to infect host cells, and promote their ingestion and subsequent destruction by phagocytes.
• Phagocytes ingest microbes and kill them, and helper T cells enhance the microbicidal abilities of the phagocytes.
• Cytotoxic T lymphocytes (CTLs) destroy cells infected by microbes that are inaccessible to antibodies and phagocytic destruction.
The goal of the adaptive response is to activate one or more of these defense mechanisms against diverse microbes that may be in different anatomic locations, such as intestinal lumens, the circulation, or inside cells.
All adaptive immune responses develop in steps, each of which corresponds to particular reactions of lymphocytes (Fig. 1-6). We start this overview of adaptive immunity with the first step, which is the recognition of antigens.
Because the number of naive lymphocytes specific for any antigen is very small (on the order of 1 in 105 or 106 lymphocytes) and the quantity of the available antigen may also be small, special mechanisms are needed to capture microbes, to concentrate their antigens in the correct location, and to deliver the antigens to specific lymphocytes. Dendritic cells are the APCs that display microbial peptides to naive CD4+ and CD8+ T lymphocytes and initiate adaptive immune responses to protein antigens. Dendritic cells located in epithelia and connective tissues capture microbes, digest their proteins into peptides, and express on their surface peptides bound to MHC molecules, the specialized peptide display molecules of the adaptive immune system. Dendritic cells carry their antigenic cargo to draining lymph nodes and take up residence in the same regions of the nodes through which naive T lymphocytes continuously recirculate. Thus, the chance of a lymphocyte with receptors for an antigen finding that antigen is greatly increased by concentrating the antigen in recognizable form in the correct anatomic location. Dendritic cells also display the peptides of microbes that enter other lymphoid tissues, such as the spleen.
Intact microbes or microbial antigens that enter lymph nodes and spleen are recognized in unprocessed (native) form by specific B lymphocytes. There are also specialized APCs that display antigens to B lymphocytes.
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