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Steps in Antibody Production

B Cell Activity

1. Antigen-bearing agents enter tissues.

2. B cell becomes activated when it encounters an antigen that fits its antigen receptors, either alone or more often in conjunction with helper T cells.

3. Activated B cell proliferates, enlarging its clone.

4. Some of the newly formed B cells differentiate further to become plasma cells.

5. Plasma cells synthesize and secrete antibodies whose molecular structure is similar to the activated B cell's antigen receptors.

6. Antibodies combine with antigen-bearing agents, helping to destroy them.

T Cell Activity

1. Antigen-bearing agents enter tissues.

2. Accessory cell, such as a macrophage, phagocytizes antigen-bearing agent, and the macrophage's lysosomes digest the agent.

3. Antigens from the digested antigen-bearing agents are displayed on the surface membrane of the accessory cell.

4. Helper T cell becomes activated when it encounters a displayed antigen that fits its antigen receptors.

5. Activated helper T cell releases cytokines when it encounters a B cell that has previously combined with an identical antigen-bearing agent.

6. Cytokines stimulate the B cell to proliferate.

7. Some of the newly formed B cells differentiate into antibody-secreting plasma cells.

8. Antibodies combine with antigen-bearing agents, helping to destroy them.

immune response may include several types of antibodies manufactured against a single microbe or virus. This is called a polyclonal response. Clinical Application 16.1 discusses how researchers use clones of single B cells to produce single, or monoclonal, antibodies.

The human body can manufacture an apparently limitless number of different antibodies, but we have a limited number of antibody genes. During the early development of B cells, sections of their antibody genes move to other chromosomal locations, creating new genetic instructions for antibodies. The great diversity of antibody types is increased further because different antibody protein subunits combine. Antibody diversity is like using the limited number of words in a language to compose an infinite variety of stories.

Antibody Molecules

Antibodies are soluble, globular proteins that constitute the gamma globulin fraction of plasma proteins (see chapter 14, p. 560).

Light chain

Light chain

Disulfide bonds

Variable region

Constant region

Figure 16.20

An immunoglobulin molecule consists basically of two identical light chains of amino acids and two identical heavy chains of amino acids.

Variable region

Disulfide bonds

Constant region

Figure 16.20

An immunoglobulin molecule consists basically of two identical light chains of amino acids and two identical heavy chains of amino acids.

Each antibody or immunoglobulin molecule is composed of four chains of amino acids that are linked together by pairs of sulfur atoms that are attached to each other by disulfide bonds. Two of these amino acid chains are identical light chains (L-chains), and two are identical heavy chains (H-chains). The heavy chains contain about twice as many amino acids as the light chains. The five major types of immunoglobulin molecules are distinguished by a particular kind of heavy chain.

As with other proteins, the sequences of amino acids of the heavy and light chains confer the unique, three-dimensional structure (conformation) of each im-munoglobulin. This special conformation, in turn, imparts the physiological properties of the molecule. For example, one end of each of the heavy and light chains consists of variable sequences of amino acids (variable regions). These regions are specialized to react with the shape of a specific antigen molecule.

Antibodies can bind to certain antigens because of the conformation of the variable regions. The antibody contorts to form a pocket around the antigen. These specialized ends of the antibody molecule are called antigen-binding sites, and the particular parts that actually bind the antigen are called idiotypes.

The remaining portions of the chains are termed constant regions because their amino acid sequences are very similar from molecule to molecule. Constant regions impart other properties of the immunoglobulin molecule, such as its ability to bond to cellular structures or to combine with certain chemicals (fig. 16.20).

Types of Immunoglobulins

Of the five major types of immunoglobulins, three constitute the bulk of the circulating antibodies. They are im-munoglobulin G, which accounts for about 80% of the antibodies; immunoglobulin A, which makes up about 13%; and immunoglobulin M, which is responsible for

Shier-Butler-Lewis: I IV. Transport I 16. Lymphatic System and I I © The McGraw-Hill

Human Anatomy and Immunity Companies, 2001

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