X - Immunology of Horses and Donkeys

Abstract

Horses and donkeys play diverse and changing roles in society around the world. Immunoglobulin classes of horses are complex and include at least four IgG subisotypes (IgGa, b, c, IgG[T]), IgA, IgM, and IgE. Four IgG subisotypes have been well described, of which three are designated as IgGa, IgGb, and IgGc on the basis of their increasing anodal mobility in immunoelectrophoresis. While the separation of IgGa from IgGb is possible in donkeys, it has been proven difficult in horses because of the similarity in the charge of these two molecules. Recently, the separation of IgGa and IgGb in horses has been described by using protein A and protein G affinity chromatography, or a combination of ion-exchange chromatography and affinity chromatography. The existence of equine IgA was first discovered when an immunological cross-reaction was observed between an immunoglobulin in equine serum and milk with antihuman alpha-chain antiserum. The molecular mass of serum IgA extends from 150 kDa to about 700 kDa with the majority being about 350 kDa, which suggests a predominant dimeric form in serum similar to cows and pigs but distinct from human serum IgA that exists largely as monomers. IgM exists in serum as a single subisotype in a pentameric structure with a molecular mass of 990 kDa, which after reduction and alkylation separates into five monomeric subunits, each capable of antigen binding but with no precipitating and agglutinating properties.