Porth's Essentials of Pathophysiology, 4e

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Infection and Immunity

U N I T 4

Virus antigen

APC

MHC-II

MHC-II

TCR

T- helper cell

CD4

Antigenic determinant or 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

Antibody

CD8

FIGURE 15-7. Pathway for immune cell participation in an adaptive immune response. APC, antigen- presenting cell; MHC, major histocompatibility complex;TCR,T-cell receptor.

when they are presented with the foreign antigen pep- tide associated with the class I MHC molecule. During a typical viral infection of a cell, small peptides from degraded viral proteins associate with MHC-I molecules and are then transported to the infected cell membrane. This complex communicates to the cytotoxic T cell that the cell must be destroyed for the overall survival of the host. The MHC-II molecule’s binding cleft accommo- dates a fragment of antigen from pathogens that have been engulfed and digested by an antigen-presenting cell during the process of phagocytosis. The engulfed patho- gen is degraded into peptides in cytoplasmic vesicles and

then complexed with the MHC-II molecule. Helper T cells recognize these complexes on the surface of antigen- presenting cells and become activated. These triggered helper T cells multiply quickly and direct other immune cells to respond to the invading pathogen through the secretion of cytokines. Each individual has a unique collection of MHC pro- teins, and a variety of MHC molecules can exist in a pop- ulation. Thus, MHC molecules are both polygenic and polymorphic. The MHC genes are the most polymorphic genes known. Because of the number of MHC genes and the possibility of several alleles for each gene, it is almost