HUMAN IMMUNOGLOBULIN CLASSES, SUBCLASSES, TYPES AND SUBTYPES
A. Immunoglobulin classes
The immunoglobulins can be divided into five different classes, based on differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a given class will have very similar heavy chain constant regions. These differences can be detected by sequence studies or more commonly by serological means (i.e. by the use of antibodies directed to these differences).
1. IgG - Gamma heavy chains
2. IgM - Mu heavy chains
3. IgA - Alpha heavy chains
4. IgD - Delta heavy chains
5. IgE - Epsilon heavy chains
B. Immunoglobulin Subclasses
The classes of immunoglobulins can de divided into subclasses based on small differences in the amino acid sequences in the constant region of the heavy chains. All immunoglobulins within a subclass will have very similar heavy chain constant region amino acid sequences. Again these differences are most commonly detected by serological means.
1. IgG Subclasses
a) IgG1 - Gamma 1 heavy chains
b) IgG2 - Gamma 2 heavy chains
c) IgG3 - Gamma 3 heavy chains
d) IgG4 - Gamma 4 heavy chains
2. IgA Subclasses
a) IgA1 - Alpha 1 heavy chains
b) IgA2 - Alpha 2 heavy chains
C. Immunoglobulin Types
Immunoglobulins can also be classified
by the type of light chain that they have. Light chain types are based
on differences in the amino acid sequence in the constant region of the
light chain. These differences are detected by serological means.
1. Kappa light chains
2. Lambda light chains
D. Immunoglobulin Subtypes
The light chains can also be
divided into subtypes based on differences in the amino acid
sequences in the constant region of the light chain.
1. Lambda subtypes
a) Lambda 1
b) Lambda 2
c) Lambda 3
d) Lambda 4
Immunoglobulins are named based on the class,
or subclass of the heavy chain and type or subtype of light chain.
Unless it is stated precisely you are to assume that all subclass,
types and subtypes are present. IgG means that all subclasses and
types are present.
Immunoglobulins considered as a population
of molecules are normally very heterogeneous because they are
composed of different classes and subclasses each of which has
different types and subtypes of light chains. In addition, different
immunoglobulin molecules can have different antigen binding
properties because of different VH and VL
VII. STRUCTURE AND SOME PROPERTIES OF IG CLASSES AND |
The structures of the IgG subclasses are
presented in Figure 7. All IgG's are monomers (7S
immunoglobulin). The subclasses differ in the number of
disulfide bonds and length of the hinge region.
Most versatile immunoglobulin because it is
capable of carrying out all of the functions of immunoglobulin
a) IgG is the major Ig in serum - 75% of serum Ig is IgG
b) IgG is the major Ig in extra vascular spaces
c) Placental transfer - IgG is the only class of Ig that
crosses the placenta. Transfer is mediated by receptor on
placental cells for the Fc region of IgG. Not all subclasses
cross equally; IgG2 does not cross well.
d) Fixes complement - Not all subclasses fix equally well;
IgG4 does not fix complement
e) Binding to cells - Macrophages, monocytes, PMN's and some
lymphocytes have Fc receptors for the Fc region of IgG. Not all
subclasses bind equally well; IgG2 and IgG4 do not bind to Fc
receptors. A consequence of binding to the Fc receptors on PMN's,
monocytes and macrophages is that the cell can now internalize
the antigen better. The antibody has prepared the antigen for
eating by the phagocytic cells. The term opsonin is used
to describe substances that enhance phagocytosis. IgG is a good
opsonin. Binding of IgG to Fc receptors on other types of cells
results in the activation of other functions.
continue to part-2
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