Porth's Pathophysiology, 9e

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UNIT IV Infection, Inflammation, and Immunity

body, specific T cells become activated and specific B cells are stimulated to produce antibodies. Once the first encounter occurs, these cells can exactly recognize a particular microor- ganism or foreign molecule because each lymphocyte is capa- ble of targeting a specific antigen and differentiating the invader from self or from other substances that may be similar to it. Cell-mediated and humoral immunity is capable of respond- ing to millions of antigens each day because there is an enor- mous variety of lymphocytes that have been programmed and selected during cellular development. Once the invading sub- stance or organism has been removed from the body, the lym- phocytes “remember” the presenting antigen and can respond rapidly during the next encounter. These ­lymphocytes are called “memory” T and B lymphocytes. They remain in the body for a

longer period of time than their predecessors and as a result can respond more rapidly on repeat exposure. The immune system usually can respond to commonly encountered microorganisms so efficiently that we are unaware of the response. B and T lymphocyte activation is triggered by anti- gen presentation to unique surface receptors (Fig. 13.6). The antigen receptor present on the B lymphocyte consists of ­membrane-bound Ig molecules that can bind a specific epitope. However, in order for B lymphocytes to produce antibodies, they require the help of specific T lymphocytes, called helper T cells . While the B lymphocytes bind to one determinant (or hapten) on an antigen molecule, the antigen-­specific helper T cell recognizes and binds to another determinant known as the “carrier.” The carrier is an APC, which has previously picked

Virus antigen

APC

MHC-II

MHC-II

TCR

T- helper cell

CD4

Antigen epitope

Activation

CD4

TCR

Cytokines

B cell

CD4

T- helper cell

T-helper memory cell

Cytokines

TCR

Cytotoxic T cell

MHC-I with viral epitope

TCR

Memory B cell

Cell death

CD8

Target cell

Plasma cell

CD8

Cytotoxic T memory cell

FIGURE 13.6  •  Pathway for imm­ une cell participation in an immune response. (APC, antigen-presenting cell; MHC, major histocompatibility complex; TCR, T-cell receptor.)

Antibody

CD8