Page 537 - Review of Medical Microbiology and Immunology ( PDFDrive )

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526
PART VII Immunology
IgG is the predominant antibody in the secondary
response and constitutes an important defense against bac-
IgM is the main immunoglobulin produced early in the
teria and viruses (See Table 59–1). IgG is the only antibody
primary response. It is present as a monomer on the sur-
to cross the placenta; only its Fc portion binds to receptors
face of virtually all B cells, where it functions as an anti-
on the surface of placental cells. This receptor, called FcRn,
4
gen-binding receptor. In serum, it is a pentamer
transports maternal IgG across the placenta into the fetal
blood. IgG is therefore the most abundant immunoglobu-
ing) chain (see Figure 59–4). IgM has a μ heavy chain.
lin in newborns. This is an example of passive immunity as
Because the pentamer has 10 antigen-binding sites, it is
the IgG is made by the mother, not by the fetus (see Chap- composed of five H2L2 units plus one molecule of J (join-
the most efficient immunoglobulin in agglutination,
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ter 57). Another important attribute of IgG is that it is one
complement fixation (activation), and other antibody
of the two immunoglobulins that can activate complement;
reactions and is important in defense against bacteria and
IgM is the other (see Chapter 63).
IgG is the immunoglobulin that opsonizes. It can opso-
tions. It has the highest avidity of the immunoglobulins;
nize (i.e., enhance phagocytosis) because there are recep-
its interaction with antigen can involve all 10 of its bind-
tors for the γH chain on the surface of phagocytes. IgM
ing sites.
does not opsonize directly, because there are no receptors
on the phagocyte surface for the μH chain. However, IgM
IgD
activates complement, and the resulting C3b can opsonize
because there are receptors for C3b on the surface of
phagocytes. Note that there are four subclasses of IgG. Sub-
may function as an antigen receptor; it is present on the
classes IgG1 and IgG3 are more effective opsonizers than
surface of many B lymphocytes. It is present in small
are IgG2 and IgG4. This immunoglobulin has no known antibody function but
amounts in serum.
IgG has various sugars attached to the heavy chains,
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especially in the CH2 domain. The medical importance
IgE
of these sugars is that they determine whether IgG will
have a proinflammatory or anti-inflammatory effect. For
example, if the IgG molecule has a terminal N-acetyl
immediate (anaphylactic) hypersensitivity (see Chapter 65),
glucosamine, it is proinflammatory because it will bind
and (2) it participates in host defenses against certain
to mannose-binding ligand and activate complement
parasites (e.g., helminths [worms]) (see Chapter 56). The
(see Chapter 63 and Figure 63–1). In contrast, if the IgG
Fc region of IgE binds to the surface of mast cells and
has a sialic acid side chain, then it will not bind and
basophils. Bound IgE serves as a receptor for antigen
becomes anti-inflammatory. Thus IgG proteins specific for
(allergen). When the antigen-binding sites of adjacent
a single antigen that are made by a single plasma cell can,
IgEs are cross-linked by allergens, several mediators are
at various times, possess different properties depending on
released by the cells, and immediate (anaphylactic) hyper-
these sugar modifications.
sensitivity reactions occur (see Figure 65–1). Although
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IgE is present in trace amounts in normal serum (approx-
mebooksfree.com mebooksfree.com mebooksfree.com nal secretions. IgE does not fix complement and does not mebooksfree.com
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IgA
imately 0.004%), persons with allergic reactivity have
IgA is the main immunoglobulin in secretions such as
greatly increased amounts, and IgE may appear in exter-
colostrum, saliva, tears, and respiratory, intestinal, and
genital tract secretions. It prevents attachment of microor-
cross the placenta.
ganisms (e.g., bacteria and viruses) to mucous membranes.
IgE is the main host defense against certain important
Each secretory IgA molecule consists of two H2L2 units
helminth (worm) infections, such as Strongyloides,
3
plus one molecule each of J (joining) chain and secretory
Trichinella, Ascaris, and the hookworms Necator and
component (Figure 59–4). The two heavy chains in IgA are
Ancylostoma. The serum IgE level is usually increased in
α heavy chains.
The secretory component is a polypeptide synthesized
ingested by phagocytes, they are killed by eosinophils
by epithelial cells that provides for IgA passage to the
that release worm-destroying enzymes. IgE specific for
mucosal surface. It also protects IgA from being degraded these infections. Because worms are too large to be
worm proteins binds to receptors on eosinophils, trigger-
in the intestinal tract. In serum, some IgA exists as mono-
ing the antibody-dependent cellular cytotoxicity (ADCC)
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mebooksfree.com mebooksfree.com mebooksfree.com 4 chains, but the heavy chain of the surface IgM has a hydrophobic mebooksfree.com
meric H2L2.
response.
3
Only IgA and IgM have J chains. Only these immunoglobulins exist as
The surface monomer IgM and the serum IgM both have μ heavy
multimers (dimers and pentamers, respectively). The J chain initiates the
sequence that mediates binding within the cell membrane, whereas the
polymerization process, and the multimers are held together by disulfide
serum IgM does not.
bonds between their Fc regions.
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