Question

You have obtained an antiserum (a polyclonal antibody preparation) that was generated by injecting a rabbit with IgA from a ferret.  The rabbit was immunized three times with the ferret IgA in monthly intervals, and a month after the final immunization, a blood sample was taken and the antiserum was collected.  At first you are pleased - this anti-IgA antiserum binds IgA present in mucosal secretions and breastmilk of ferrets as well as IgA in their blood.  However, further testing reveals that it also binds ferret IgG, IgM, IgE and IgD!  What is the most likely explanation for this binding activity?  Why is this anti-IgA antiserum binding all of the other isotypes and not just IgA?  Aren’t antibodies supposed to be very specific?

Answer 1

Polyclonal antibodies are antibodies which have been derived from multiple B cell or cell lineages . The antibodies that are polyclonal recognize different epitopes and have different degrees of specificity.

Epitopes are antigenic determinants i.e. the part of the antigen that is recognised by the immune system ( specifically by antibodies, B cells or T cells). In simple words, epitope is a small site on an antigen to which a complementary antibody may specifically bind. This is usually one to six monosaccharides or five to eight amino acid residues on the surface of antigen.

Serum contains a mixture of anti-glycan antibodies that can recognize the same antigen and competition for binding can potentially influence the detection of antibody subpopulations that are more relevant to disease process. The most abundant antibody isotypes in serum are IgG, IgM and IgA and these isotypes compete for the same glycan antigen. While IgG and IgA antibodies outcompete IgM for "peptide or protein antigens", IgM outcompetes IgG and IgA for many "glycan antigens". Antibodies to a particular glycan may vary in terms of affinity,specifity, concentration and isotype but all compete for binding to the same antigen. As a result, binding of one can influence detection of the others. Due to affinity maturation and class switching process, IgG and IgA antibodies have higher affinity than IgM antibodies. Whereas, with more binding sites, carbohydrate-binding IgM antibodies outcompete IgG and IgA antibodies through enhanced avidity. IgD and IgE are also present in serum at low concentrations and are capable of competing.