Antibodies Immunoglobulins
Immunoglobulins (Igs) are glycoproteins. Each molecule of Ig
consists of two pairs of polypeptide chains of different sizes. The smaller
chains are called ‘light’ (L) chains and the larger ones are called ‘heavy’
(H) chains.
The L chain has a molecular weight of approximately 25,000 and
the H chains is of 50,000. The L chain is attached to the H chain by a
disulphide bond. The two H chains are also joined together by
S-S bonds, depending on the class of immunoglobulins. The H chains are
structurally and antigenically distinct for each class and are designated by the
Greek letter corresponding to the immunoglobulin class, as follows :
IgG (Gamma) IgA (alpha ) IgM(mu) Igd (delta) and IgE (epsilon)
The L chains are similar in all classes of immunoglobulins. They
occur in two varieties, kappa (k) and lambda (l). A molecule of
immunoglobulin may have either kappa or lambda chains, but never both.
Region of polypeptide chains
Each heavy and light chains consists of two regions viz.,
1.The variable (V) region or Fab region: The V region shows a wide
variation in amino acid sequences in the amino or N-terminal portion of the
molecule. These areas of high variability in the variable region of H and L
chains are called ‘hotspots’ or hypervariable regions. These hotspots are
most intimately involved in the information of the antigen-binding site. In
both VH and VL regions of the chain atleast three hypervariable regions/
hotspots are present. The infinite range of the antibody specificity of
immunoglobins depends on the variability of the amino acid sequences at the
‘variable regions’ of the H and L chains, which form the antigen combining
sites (Paratope). Immunoglobulins
2. Constant (c) region (or) Fc region (Fc = fragment of constant region) :
The C region denotes constant region with unvarying amino acid sequence in
the C or COOH terminal portion of the molecule.
(Fab – Fragment of antigen binding site
Fc – Fragment of constant region)
Depending on the observable reaction produced on mixing with antigens,
the antibodies are designated variously as agglutinins,
precipitins and so on. Sera having high antibody levels following infection
or immunization are called immune sera or anti sera. Immunoglobulins
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