Introduction to Immunological methods

Antibodies are proteins made by the immune system of animals as part of a defense system  against infection  by foreign organisms.  The immune system must be able to distinguish between “self” and “non-self”, and to  eliminate  the  latter,  and antibodies play  a role  in this  process. Because of their specificity and versatility, antibodies are  also very  useful reagents in the  identification  and analysis of proteins and it is largely in this light - as useful reagents.

The structure of antibodies

Antibodies are a class  of blood proteins  known  as γ-globulin . The most common γ-globulin is the  IgG type,  which consists  of four peptide chains, two heavy and two light, held together by disulfide bonds.  A schematic sketch of the structure is shown in Fig. 1.

Figure 1. A simplified schematic representation of the structure of an IgG antibody.

The IgG molecule can be cleaved in the hinge region by papain, to yield three fragments,  two  F_ab fragments and one Fc fragment. “F _ab”  stands for “fragment, antigen binding” and this reflects the fact that the outer aspect of the heavy and light chains, in the  F_ab fragment, contain so-called hypervariable regions which constitute  the  antigen  binding site.

The hypervariable regions are different in the  IgG molecules synthesized by different  plasma  cell  clones,  but will  be identical  in  every  molecule originating from a single clone of plasma cells.  The hypervariable  regions are constituted of loops  at  one  end of a fl-barrel structure, Such loops can vary without disturbing the underlying stability of the barrel structure.  “F _c” stands for  “fragment,  crystallizable” and this reflects the fact that the F_c fragment is invariant and therefore  can be crystallized, even when it is derived from a polyclonal antiserum. In mammals, antibodies are transferred to the neonate in the form of colostrum, which is the  first milk produced in the  early post-partum stage.  In birds the transfer of antibodies occurs via the  egg yolk  and egg yolk thus provides  a convenient  source from  which antibodies may be isolated. The antibodies from egg yolk have a structure similar, but not identical, to IgG and are known as IgY antibodies.

References 

Dennison, C. (2002). A guide to protein isolation . School of Molecular mid Cellular Biosciences, University of Natal . Kluwer Academic Publishers new york, Boston, Dordrecht, London, Moscow .