T Cells and the Cellular Immune Response

T cells attach to foreign, antigen-bearing cells, such as bacterial cells, and interact directly—that is, by cell-to-cell contact. This is called the cellular immune response, or cell-mediated immunity.

T cells (and some macrophages) also synthesize and secrete polypeptides called cytokines (or more specifically, lymphokines) that enhance certain cellular responses to antigens. For example, interleukin-1 and interleukin-2 stimulate synthesis of several cytokines from other T cells. In addition, interleukin-1 helps activate T cells, whereas interleukin-2 causes T cells to proliferate and activates a certain type of T cell (cytotoxic T cells). Other cytokines called colony stimulating factors (CSFs) stimulate production of leukocytes in the red bone marrow, cause B cells to grow and mature, and activate macrophages. Certain cy-tokine combinations shut off the immune response. Table 16.4 summarizes several cytokine types.

T cells may also secrete toxins that kill their antigen-bearing target cells: growth-inhibiting factors that prevent target cell growth, or interferon that inhibits proliferation of viruses and tumor cells. Several types of T cells have distinct functions.

T Cell Types

Helper T cells mobilize the immune system to stop a bacterial infection through a complex series of steps. First, a macrophage phagocytizes a bacterium, digesting it in its

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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