Page 309 - Color Atlas of Biochemistry

P. 309

300 Tissues and organs

Antibodies Disulfide bonds link the two heavy chains to
each other and also link the heavy chains to
Soluble antigen receptors, which are formed the light chains. Inside the domains, there are
by activated B cells (plasma cells; see p. 294) also disulfide bonds that stabilize the tertiary
and released into the blood, are known as structure. The domains are approximately 110
antibodies. They are also members of the im- amino acids (AA) long and are homologous
munoglobulin family (Ig; see p. 296). Anti- with each other. The antibody structure evi-
bodies are an important part of the humoral dently developed as a result of gene duplica-
immune defense system. They have no anti- tion. In its central region, known as the
microbial properties themselves, but support “hinge” region, the antibodies are highly mo-
the cellular immune system in various ways: bile.
1. They bind to antigens on the surface of
pathogens and thereby prevent them from
interacting with body cells (neutralization; B. Classes of immunoglobulins
see p. 404, for example). Human immunoglobulins are divided into
2. They link single-celled pathogens into five classes. IgA (with two subgroups), IgD,
aggregates (immune complexes), which are IgE, IgG (with four subgroups), and IgM are
more easily taken up by phagocytes (aggluti- defined by their H chains, which are desig-
nation). nated by the Greek letters α, δ, ε, γ,and µ.By
3. They activate the complement system contrast, there are only two types of Lchain
(see p. 298) and thereby promote the innate (κ and λ). IgD and IgE (like IgG) are tetramers
immune defense system (opsonization). with the structure H 2 L 2 . By contrast, soluble
In addition, antibodies have become indis- IgA and IgM are multimers that are held
pensable aids in medical and biological diag- together by disulfide bonds and additional
nosis (see p. 304). Jpeptides (joining peptides).
The antibodies have different tasks. IgMs
are the first immunoglobulins formed after
A. Domain structure of immunoglobulin G
contact with a foreign antigen. Their early
Type G immunoglobulins (IgG)are quantita- forms are located on the surface of B cells
tively the most important antibodies in the (see p. 296), whilethe later forms arese-
blood, where they form the fraction of γ-glob- creted from plasma cells as pentamers. Their
ulins(see p. 276). IgGs (mass150 kDa) are action targets microorganisms in particular.
tetramers with two heavy chains (H chains; Quantitatively, IgGs are the most important
redororange) andtwo light chains (L chains; immunoglobulins (see the table showing se-
yellow). Both H chains are glycosylated (vio- rum concentrations). They occur in the blood
let; see also p. 43). and interstitial fluid. As they can pass the
The proteinase papain cleaves IgG into two placenta with the help of receptors, they can
F ab fragments and one F c fragment. The F ab be transferred from mother to fetus. IgAs
(“antigen-binding”) fragments, which each mainly occur in the intestinal tract and in
consist of one L chain and the N-terminal body secretions. IgEs are found in low con-
part of an H chain, are able to bind antigens. centrations in the blood. As they can trigger
The F c (“crystallizable”) fragment is made up degranulation of mast cells (see p. 380), they
of the C-terminal halves of the two H chains. play an important role in allergic reactions.
This segment serves to bind IgG to cell sur- The function of IgDs is still unexplained. Their
faces, for interaction with the complement plasma concentration is also very low.
system and antibody transport.
Immunoglobulins are constructed in a
modular fashion from several immunoglobu-
lin domains (shown in the diagram on the
right in Ω form). The H chains of IgG contain
four of these domains (V H , C H 1, C H 2, and
C H 3) and the L chains contain two (C L and V L ).
The letters C and V designate constant or
variable regions.

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