The Immune Response

When a foreign substance - a virus, a bacterium, or even a foreign protein-invades the tissues of a higher vertebrate (like a human), the organism defends itself by what is called the immune response. The immune response is a first line of defense against infection and probably against cancer cells as well. Crippling of the immune system by the HIV virus makes AIDS (acquired immune deficiency syndrome) a disease that has so far proved to be almost invariably fatal. The immune response has two facets:

> 1. Humoral immune response - Lymphatic cells called B lymphocytes synthesize specific immunoglobulin molecules that are excreted from the cell and bind to the invading substance. Binding either precipitates the foreign substance or marks it for destruction by cells called macrophages.

> 2. Cellular immune response - Lymphatic cells called T lymphocytes, bearing immunoglobulin-like molecules on their surfaces, recognize and kill foreign or aberrant cells.

Antigens and Antibodies - The foreign substance that elicits an immune response is called the antigen. A specific immunoglobulin that binds to the antigen is called the antibody. If the invading particle is large, like a cell, a virus, or a protein, many different antibodies may be elicited, each type binding specifically to an antigenic determinant (or epitope) on the surface of the particle (Figure 7.29a).

The immune response is incredibly versatile:

> 1. It can respond to an enormous number of different foreign substances.

> 2. The immune response has a so-called memory: After an initial exposure to a given antigen, a second exposure at a later date results in rapid and much more massive production of the antibodies specific to the antigen.

Clonal Selection Theory - The clonal selection theory explains how the body has an inherent ability to produce an immense diversity of antibodies with different amino acid sequences that are able to bind an enormous range of antigens. The basic postulates are illustrated in Figure 7.30.

See also: Clonal Selection Theory, Antibody Structure, Generation of Antibody Diversity, Antibody Diversity (from Chapter 25) T Cells and the Cellular Response, AIDS and the Immune Response

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