Interactions Between Antigen Presenting Cells and T Lymphocytes

The major histocompatibility complex of genes produces two classes of MHC molecules, designated class 1 and class 2, that are found on the cell surface. The class-1 molecules are produced by all cells in the body except red blood cells. Class-2 MHC molecules are produced only by antigen-presenting cells—macrophages, dendritic cells, and B lymphocytes. These cells present their class-2 MHC molecules together with the foreign polypeptide antigen to helper T lymphocytes. This activates the helper T lymphocytes, so that they can promote the B-cell immune response.

The helper T lymphocytes can only be activated by antigens presented to them in association with class-2 MHC molecules. Killer (cytotoxic) T lymphocytes, by contrast, can be activated to destroy a victim cell only if the cell presents antigens to them in association with class-1 MHC molecules. The different require-

The Immune System 46I

Heiper T ceii

Kiiier T ceii

CD4-

coreceptor

T cell receptor Foreign antigen

Class-2 MHC molecule

Kiiier T ceii

T cell receptor Foreign antigen

CD8 coreceptor

Class-1 MHC molecule

Target ceii

Antigen-presenting cell

Class-1 MHC molecule

Target ceii

CD8 coreceptor

â–  Figure I5.I6 Coreceptors on helper and killer T cells. A foreign antigen is presented to T lymphocytes in association with MHC molecules. The CD4, on helper T cells, and CD8 coreceptors on killer T cells, permit each type of T cell to interact only with a specific class of MHC molecule.

ments for class-1 or class-2 MHC molecules result from the presence of coreceptors, which are proteins associated with the T cell receptors. The coreceptor known as CD8 is associated with the killer T lymphocyte receptor and interacts only with the class-1 MHC molecules; the coreceptor known as CD4 is associated with the helper T lymphocyte receptor and interacts only with the class-2 MHC molecules. These structures are illustrated in figure 15.16.

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