Antibody Structure

All antibody molecules consist of four interconnected polypep-tide chains. Two long, heavy chains (the H chains) are joined to two shorter, lighter L chains. Research has shown that these four

Test Yourself Before You Continue

1. List the phagocytic cells found in blood and lymph, and indicate which organs contain fixed phagocytes.

2. Describe the actions of interferons.

3. Distinguish between innate and adaptive immunity, and describe the properties of antigens.

4. Distinguish between B and T lymphocytes in terms of their origins and immune functions.

5. Identify the primary and secondary lymphoid organs and describe their functions.

6. Describe the events that occur during a local inflammation.

Endoplasmic reticulum

Antibody

+ Antigen

Mitochondrion a

Plasma cell xyy

Plasma cell

Plasma cell xyy

Plasma cell

Memory cell Memory cell

■ Figure 15.7 B lymphocytes are stimulated to become plasma cells and memory cells. B lymphocytes have antibodies on their surface that function as receptors for specific antigens. The interaction of antigens and antibodies on the surface stimulates cell division and the maturation of the B cell progeny into memory cells and plasma cells. Plasma cells produce and secrete large amounts of the antibody. (Note the extensive rough endoplasmic reticulum in these cells.)

■ Figure 15.8 A pathogen, such as a bacterium, has many different antigens on its surface. Each of these antigens interacts with a specific B-cell receptor protein, thereby activating those B cells that can produce antibodies against those specific antigens.

B cell

B-cell receptor Surface antigen

Pathogen

B cell

B-cell receptor Surface antigen

Pathogen

The Immune System chains are arranged in the form of a Y. The stalk of the Y has been called the "crystallizable fragment" (abbreviated Fc), whereas the top of the Y is the "antigen-binding fragment" (Fab). This structure is shown in figure 15.10.

■ Figure 15.9 The separation of serum protein by electrophoresis. This technique separates different groups of proteins on the basis of their electric charges and sizes. (A = albumin; ai = alpha-1 globulin; a2 = alpha-2 globulin; P = beta globulin; f = gamma globulin.)

Table 15.6

The Immunoglobulins

Immunoglobulin

Functions

IgG

Main form of antibodies in circulation: production

increased after immunization; secreted during

secondary response

IgA

Main antibody type in external secretions, such as

saliva and mother's milk

IgE

Responsible for allergic symptoms in immediate

hypersensitivity reactions

IgM

Function as antigen receptors on lymphocyte surface

prior to immunization; secreted during primary

response

IgD

Function as antigen receptors on lymphocyte surface

prior to immunization; other functions unknown

The amino acid sequences of some antibodies have been determined through the analysis of antibodies sampled from people with multiple myelomas. These lymphocyte tumors arise from the division of a single B lymphocyte, forming a population of genetically identical cells (a clone) that secretes identical antibodies. Clones and the antibodies they secrete are different, however, from one patient to another. Analyses of these antibodies have shown that the Fc regions of different antibodies are the same (are constant), whereas the Fab regions are variable. Variability of the antigen-binding regions is required for the specificity of antibodies for antigens. Thus, it is the Fab region of an antibody that provides a specific site for bonding with a particular antigen (fig. 15.11).

B lymphocytes have antibodies on their plasma membrane that serve as receptors for antigens. Combination of antigens with these antibody receptors stimulates the B cell to divide and produce more of these antibodies, which are secreted. Exposure to a given antigen thus results in increased amounts of the specific type of antibody that can attack that antigen. This provides active immunity, as described in the next major section.

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