Info

Cytokine

Function

Colony stimulating factors

Stimulate bone marrow to produce lymphocytes

Interferons

Block viral replication, stimulate macrophages to engulf viruses, stimulate B cells to produce antibodies, attack cancer cells

Interleukins

Control lymphocyte differentiation and growth

Tumor necrosis factor

Stops tumor growth, releases growth factors, causes fever that accompanies bacterial infection, stimulates lymphocyte differentiation

lysosomes. Some bacterial antigens exit the lysosomes and move to the macrophage's surface. Here, they are displayed in association with certain MHC proteins. Then, a helper T cell contacts a displayed foreign antigen becoming activated if the displayed antigen fits and combines with the helper T cell's antigen receptors (fig. 16.18). Once activated, the helper T cell stimulates the B cell to produce antibodies that are specific for the displayed antigen.

A type of helper T cell called a CD4 cell is the prime target of HIV, the virus that causes AIDS. (CD4 stands for the "cluster-of-differentiation" antigen it bears that enables it to recognize a macrophage displaying a foreign antigen.) Considering the role of CD4 helper T cells as key players in establishing immunity—they stimulate B cells and secrete cytokines—it is no wonder that harming them destroys immunity.

Memory T cells are among the many T cells produced upon initial exposure to an antigen, but they include only those cells not responding to the antigen at that time. These cells provide for a no-delay response to any future exposure to the same antigen, with immediate differentiation into cytotoxic T cells. This response generally vanquishes the invading organism before it can cause the body to produce signs and symptoms of disease.

Another type of T cell is a cytotoxic T cell, which recognizes nonself antigens that cancerous cells or vi-rally infected cells display on their surfaces near certain MHC proteins. A cytotoxic T cell becomes activated when it combines with an antigen that fits its receptors. Then, the T cell proliferates, enlarging its clone of cells (identical cells originating from division of a single cell). Cytotoxic T cells then bind to the surfaces of antigen-bearing cells, where they release a protein called perforin that cuts porelike openings, destroying these cells. Another type of cell, called a natural killer cell, also uses perforin to destroy tumor cells. Both cytotoxic T cells and natural killer cells have other ways to lyse cells. Cytotoxic T cells continually monitor the body's cells, recognizing and eliminating tumor cells and cells infected with viruses.

99 How do T cells become activated? ^9 What is the function of cytokines?

^9 How do cytotoxic T cells destroy antigen-bearing cells?

Essentials of Human Physiology

Essentials of Human Physiology

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