Specific attachment to phagocytes; complement fixation; ability to cross placenta
Complement fixation; first antibody produced during the primary immune response; only class produced in response to T-independent antigens
Secreted into saliva, milk, mucus, and other secretions; secretory form resists enzymatic degradation
Attaches to mast cells and basophils; cell-bound IgE that binds antigen causes the cell to release its granule contents
Agglutination; precipitation; opsonization; ADCC; complement activation
Agglutination; precipitation; complement activation
Protection of mucous membranes by preventing attachment of organisms (mucosal immunity)
Involved with development and maturation of the antibody response; functions have not been clearly described
Involved in many allergic reactions; functions in ADCC; helps to expel parasites
Protective Outcomes of Antibody-Antigen Binding
The protective outcomes of antibody-antigen binding depend partly on the class of the antibody, and may include these mechanisms (figure 16.6):
■ Neutralization. In order to damage a host cell, toxins and viruses typically must bind specific molecules on the cell surface. A toxin or virus that has been coated with antibody is prevented from interacting with a cell, and therefore can no longer cause damage.
■ Immobilization and prevention of adherence. Binding of antibodies to surface structures such as flagella and pili on a bacterium can interfere with such functions as motility and attachment. If these abilities are necessary for the microbe's interaction with the host, binding of antibodies to these structures can protect the host.
■ Agglutination and precipitation. Antibodies can agglutinate or precipitate antigens into clumps, effectively rounding up dispersed antigens to create one large "mouthful" for a phagocytic cell. This occurs because single antibody molecules can bind adjacent antigens, interconnecting individual antigens to form a network. IgM is particularly effective at agglutinating antigens because, as a pentamer, it has 10 antigen-binding sites.
■ Opsonization. Recall from chapter 15 that antigens bound by the complement protein C3b are more easily engulfed by phagocytic cells; the C3b-coated antigens are said to be opsonized. IgG molecules have an analogous effect when they bind to antigen. Macrophages and neutrophils both have receptors for the Fc region of these antibodies on their surface, facilitating the attachment of the phagocytic cell to the antibody-coated antigen as a prelude to engulfment.
■ Complement activation. The binding of antibody to antigen can trigger the classical pathway of the complement cascade. When multiple antibodies of the classes IgG or IgM are bound to a cell surface, a specific complement protein attaches to their Fc regions, initiating the cascade. Activation of the complement system results in formation of membrane attack complexes, stimulation of the inflammatory response, and production of the opsonin C3b. ■ complement, p. 381
400 Chapter 16 The Adaptive Immune Response
Figure 16.5 Model of an IgG Molecule The inset shows that each L chain consists of two domains, and each H chain of four domains. In the model, one H chain is red and the other is blue; one L chain is green and the other is yellow. A complex sugar that is part of the antibody is gray.
■ Antibody-dependent cellular cytotoxicity (ADCC).
When multiple IgG molecules bind to a cell, that cell becomes a target for destruction by certain cells. For example, natural killer (NK) cells can attach to the Fc regions of those antibodies and, once attached, release compounds directly to the target cell, killing that cell. The mechanism by which NK cells destroy target cells will be described later. ■ natural killer cells, pp. 379,411
Antibodies are routinely referred to by their class, such as IgM or IgG, but they may also be known by their protective outcome. For example, an IgG antibody that causes the antigen to agglutinate is an agglutinin; one that causes it to precipitate is a precipitin; one that coats the antigen so that it is more readily phagocytized is an opsonin; one that leads to complement lysis is a complement-fixing antibody; one that neutralizes a toxin is an antitoxin; and one that neutralizes the ability of a virus to infect cells is a neutralizing antibody.
All five major classes of immunoglobulin molecules have the same basic structure: two identical light chains connected by disulfide bonds to two identical heavy chains. Characteristics that distinguish each class structurally are the amino acid sequences, the number of domains, and the carbohydrate content of the constant portion of the heavy chains (see table 16.1). As mentioned previously, some of the immunoglobulins can form multimers of the basic structure.
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