Treatment with the enzyme papain gives rise to one double antigen-binding
fragment(F(ab^0 ) 2 )and multiple Fc fragments. The enzyme digests the Fc until it
reaches the hinge region of the antibody which is protected by a disulphide bond.
F(ab^0 ) 2 fragments have two binding sites and can be used in place of native antibody
molecules.
There are five major classes of antibody molecule, also known as immunoglobulins
(Ig); G is the commonest and it is characterised by its Y-shaped structure. The other
classes of antibody are immunoglobulins M, A, D and E (Fig. 7.3).
Immunoglobulin M (IgM)is produced early in immune responses. It is produced by
immature and newly activated B lymphocytes that have been exposed to an antigen
for the first time. It is found on the surface of B cells frequently in association with
IgD. Structurally it is formed from five immunoglobulin G molecules in a ring
complexed by a mu chain. It may also be found as a hexamer without the mu chain.
The molecule tends to have low affinity and poor avidity to antigen. It is much less
specific and will react to a range of antigens without immunisation having taken
place. It is known as ‘natural antibody’ as a result. Its production rises dramatically
after first exposure to antigen and is characteristic of theprimary immune response.
It is generally only found in serum as its large size prevents it from crossing tissue
boundaries. The pentameric form is particularly useful for complexing antigen such as
bacteria into aggregates either for disposal or for further processing by the immune
system. Cells secreting IgM can progress to IgG production, in time, if the animal is
challenged again by the antigen. This progression to IgG production is known as
affinity maturationand requires maturation of the cells to memory cell status. After
several encounters with an antigen a background level of specific antibody will be
found in the animal’s blood along with a population of B memory cells capable of
rapidly responding to its presence by initiating high levels of antibody secretion.
This status is known as immune and is the basis of both artificial immunisations
for protection against disease and also for the production of antibodies for both
diagnostic and therapeutic use. A status of hyperimmunity may be reached after
repeated exposure to an antigen leading to extremely high levels of circulatingImmunoglobulin G Immunoglobulin AImmunoglobulin E
on mast cellImmunoglobulin D
on B cellImmunoglobulin MFig. 7.3Immunoglobulin classes.271 7.1 Introduction