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following certain blood transfusions; activates complement

IgD

Surface of most B lymphocytes

B cell activation

IgE

Exocrine gland secretions

Promotes inflammation and allergic reactions

about 6%. Immunoglobulin D and immunoglobulin E account for the remainder of the antibodies.

Immunoglobulin G (IgG) is in plasma and tissue fluids and is particularly effective against bacteria, viruses, and toxins. IgG also activates a group of enzymes called complement, which is described in the following section titled "Antibody Actions."

Immunoglobulin A (IgA) is commonly found in exocrine gland secretions. It is in breast milk, tears, nasal fluid, gastric juice, intestinal juice, bile, and urine.

A newborn does not yet have its own antibodies but does retain IgG that passed through the placenta from the mother. These maternal antibodies protect the infant against some illnesses to which the mother is immune. The maternal antibody supply begins to fall just about when the infant begins to manufacture its own antibodies. The newborn also receives IgA from colostrum, a substance secreted from the mother's breasts for the first few days after birth. Antibodies in colostrum protect against certain digestive and respiratory infections.

Immunoglobulin M (IgM) is a type of antibody that develops in the plasma in response to contact with certain antigens in foods or bacteria. The antibodies anti-A and anti-B, described in chapter 14 (p. 571), are examples of IgM. IgM also activates complement.

Immunoglobulin D (IgD) is found on the surfaces of most B cells, especially those of infants. IgD acts as an antigen receptor and is important in activating B cells (see fig. 16.19).

Immunoglobulin E (IgE) appears in exocrine secretions along with IgA. It is associated with allergic reactions that are described later in this chapter in the section titled "Allergic Reactions." Table 16.6 summarizes the major immunoglobulins and their functions.

99 What is an immunoglobulin?

^9 Describe the structure of an immunoglobulin molecule. ^9 Name the five major types of immunoglobulins.

Antibody Actions

In general, antibodies react to antigens in three ways. Antibodies directly attach to antigens, activate a set of enzymes (complement) that attack the antigens, or stimulate localized changes that help prevent spread of the antigens.

Antibodies that attach to antigens cause them to clump together (agglutinate) or to form insoluble substances (precipitation). Such actions make it easier for phagocytic cells to engulf the antigen-bearing agents and eliminate them. In other instances, antibodies cover the toxic portions of antigen molecules and neutralize their effects (neutralization). However, under normal conditions, complement activation is more important in protecting against infection than direct antibody attachment.

Complement is a group of proteins (complement system) in plasma and other body fluids. When certain IgG or IgM antibodies combine with antigens, they expose reactive sites on the antibody constant regions. This triggers a series of reactions leading to activation of the complement proteins, which, in turn, produce a variety of effects, including coating the antigen-antibody complexes (op-sonization), making the complexes more susceptible to phagocytosis; attracting macrophages and neutrophils into the region (chemotaxis); clumping antigen-bearing agents; rupturing membranes of foreign cells; and altering the molecular structure of viruses, making them harmless (fig. 16.21). Other proteins promote inflammation, which helps prevent the spread of infectious agents.

Immunoglobulin E promotes inflammation that may be so intense that it damages tissues. This antibody is usually attached to the membranes of widely distributed mast cells (see chapter 5, p. 151). When antigens combine with the antibodies, the resulting antigen-antibody complexes stimulate mast cells to release biochemicals, such as histamine, that cause the changes associated with inflammation, such as vasodilation and edema. Table 16.7 summarizes the actions of antibodies.

99 In what general ways do antibodies function?

Q What is the function of complement?

H How is complement activated?

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