Page 852 - Fundamentals of anatomy physiology

Chapter 22 The Lymphatic System and Immunity 839

Table 22–1 Classes of Antibodies

Secretory
piece

IgG is the largest and most IgE attaches as an IgD is an individual IgM is the first class of antibody IgA is found primarily in
diverse class of antibodies. individual molecule to molecule on the surfaces secreted after an antigen is glandular secretions such
They account for 80 percent of the exposed surfaces of B cells, where it can encountered. IgM concentration as mucus, tears, saliva, and
all antibodies. IgG antibodies of basophils and mast bind antigens in the declines as IgG production semen. These antibodies attack
are responsible for resistance cells. When a suitable extracellular fluid. This accelerates. Although plasma pathogens before they gain
against many viruses, bacteria, antigen is bound by binding can play a role cells secrete individual IgM access to internal tissues. IgA
and bacterial toxins. These IgE molecules, the in the sensitization of molecules, IgM circulates as antibodies circulate in blood as
antibodies can cross the cell is stimulated to the B cell involved. a five-antibody starburst. The individual molecules or in pairs.
placenta, and maternal IgG release histamine and anti-A and anti-B antibodies Epithelial cells absorb them
provides passive immunity to other chemicals that responsible for the agglutination from the blood and attach a
the fetus during embryological accelerate inflammation of incompatible blood types are secretory piece, which confers
development. However, the in the immediate area. IgM antibodies. IgM antibodies solubility, before secreting
anti-Rh antibodies produced IgE is also important in may also attack bacteria that are the IgA molecules onto the
by Rh-negative mothers the allergic response. insensitive to IgG. epithelial surface.
are also IgG antibodies and
produce hemolytic disease of
the newborn.

The formation of an antigen–antibody complex may cause molecule change shape. This change exposes areas that
the elimination of the antigen in seven ways:
bind complement proteins. The bound complement mole-
1. Neutralization. Both viruses and bacterial toxins have specific
sites that must bind to target regions on body cells before cules then activate the complement system, which destroys
they can enter or injure those cells. Antibodies may bind to
those sites, making the virus or toxin incapable of attaching the antigen (as discussed previously). 22
itself to a cell. This mechanism is known as neutralization.
4. Attraction of Phagocytes. Antigens covered with antibodies
2. Precipitation and Agglutination. Each antibody molecule has
two antigen binding sites, and most antigens have many attract eosinophils, neutrophils, and macrophages. These
antigenic determinant sites. If individual antigens (such as
macromolecules or bacterial cells) are far apart, an antibody cells phagocytize pathogens and destroy foreign or abnor-
molecule will necessarily bind to two antigenic sites on the
same antigen. However, if antigens are close together, an an- mal plasma membranes.
tibody can bind to antigenic determinant sites on two sepa-
rate antigens. In this way, antibodies can link large numbers 5. Opsonization. A coating of antibodies and complement pro-
of antigens together. The three-dimensional structure cre- teins increases the effectiveness of phagocytosis. This ef-
ated by such binding is known as an immune complex. fect is called opsonization. Some bacteria have slick plasma
When the antigen is a soluble molecule, such as a toxin, this membranes or capsules, but opsonization makes it easier
process may create complexes that are too large to remain for phagocytes to hold onto their prey before they engulf it.
in solution. The formation of insoluble immune complexes Phagocytes can bind more easily to antibodies and comple-
is called precipitation. When the target antigen is on the ment proteins than they can to the bare surface of a pathogen.
surface of a cell or virus, the formation of large complexes is
called agglutination. For example, the clumping of eryth- 6. Stimulation of Inflammation. Antibodies may promote in-
rocytes that takes place when incompatible blood types are flammation by stimulating basophils and mast cells.
mixed is an agglutination reaction. p. 692
7. Prevention of Bacterial and Viral Adhesion. Antibodies dis-
3. Activation of the Complement System. When an antibody solved in saliva, mucus, and perspiration coat epithelia,
molecule binds to an antigen, portions of the antibody adding an additional layer of defense. A covering of an-
tibodies makes it difficult for pathogens to attach to and
penetrate body surfaces.

&T i p s T r i c k s
The classes of immunoglobulins—IgM,IgA, IgD, IgG, IgE—
spell MADGE.