Lymphatic System and Immunity
Antibody Actions
In general, antibodies react to antigens in three ways.
Antibodies directly attack antigens, activate complement, or
stimulate localized changes (infl
ammation) that help prevent
spread of the pathogen.
In a direct attack, antibodies combine with antigens
and cause them to clump (agglutinate) or to form insoluble
substances (precipitation). Such actions make it easier for
phagocytic cells to engulf the antigen-bearing pathogens and
eliminate them. In other instances, antibodies cover the toxic
parts of antigen molecules and neutralize their effects (neu-
tralization). However, under normal conditions, complement
activation is more important in protecting against infection
than is direct antibody attack.
When certain IgG or IgM antibodies combine with
antigens, they expose reactive sites on the antibody con-
stant regions. This triggers a series of reactions leading to
activation of complement proteins, which, in turn, produce
a variety of effects, including coating the antigen-antibody
complexes (opsonization), making the complexes more sus-
ceptible to phagocytosis; attracting macrophages and neutro-
phils into the region (chemotaxis); clumping antigen-bearing
cells; rupturing membranes of foreign cells (lysis); and alter-
ing the molecular structure of viruses, making them harm-
less. 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 usu-
ally attached to the membranes of widely distributed
(see chapter 5, p. 155). When antigens combine with
the antibodies, the resulting antigen-antibody complexes
stimulate mast cells to release biochemicals, such as hista-
mine, that cause the changes associated with infl ammation,
such as vasodilation and edema.
Table 16.8
summarizes the
actions of antibodies.
In what general ways do antibodies function?
How is complement activated?
What is the function of complement?
immunoglobulin A, which makes up about 13%; and immu-
noglobulin M, responsible for about 6%. The remainder of
the antibodies are immunoglobulin D or immunoglobulin E.
Immunoglobulin G
(IgG) is in plasma and tissue fl uids
and is effective against bacteria, viruses, and toxins. IgG also
activates complement proteins introduced in section 16.7
and described further in the following section “Antibody
Actions.” Anti-Rh antibodies are examples of IgG and, as
described in chapter 14 (p. 547), can cross the placenta.
Immunoglobulin A
(IgA) is in exocrine gland secretions.
It is in breast milk, tears, nasal fl uid, gastric juice, intestinal
juice, bile, and urine.
A newborn does not yet have its own antibodies but has IgG that
passed through the placenta from the mother. These maternal anti-
bodies 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 make its own antibodies. The newborn
also receives IgA from colostrum, which is secreted from the mother’s
breasts for the F
rst few days after birth. Antibodies in colostrum pro-
tect against certain digestive and respiratory infections.
Immunoglobulin M
(IgM) is a type of antibody produced
in plasma in response to contact with certain antigens in
foods or bacteria. Examples of IgM are the anti-A and anti-B
antibodies, described in chapter 14 (p. 544). IgM also acti-
vates complement.
Immunoglobulin D
(IgD) is on the surfaces of most B
cells, especially those of infants. IgD acts as an antigen recep-
tor and is important in activating B cells (see F
g. 16.18).
Immunoglobulin E
(IgE) appears in exocrine secretions
with IgA. It is associated with allergic reactions, described later
in this chapter in the section “Allergic Reactions.”
Table 16.7
summarizes the major immunoglobulins and their functions.
How are B cells activated?
How does the antibody response protect against diverse infections?
What is an immunoglobulin?
Describe the structure of an immunoglobulin molecule, and name
the F
ve major types of immunoglobulins.
Characteristics of Major Immunoglobulins
Major Function
Plasma and tissue ±
Defends against bacteria, viruses, and toxins; activates complement
Exocrine gland secretions
Defends against bacteria and viruses
Reacts with antigens on some red blood cell membranes following mismatched blood transfusions; activates
Surface of most B lymphocytes
B cell activation
Exocrine gland secretions
Promotes in±
ammation and allergic reactions
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