Serum Immunoglobulin concentrations (IgG, IgM)
Because humoral immunodeficiency is the most common form of immune disorder, the first step when there is suspicion of an underlying immunodeficiency is to measure serum IgG and IgM concentrations using immunoturbidimetric or radial immunodiffusion tests. Serum immunoglobulin concentrations reflect B cell differentiation and function in lymphoid tissues. Repeated tests should be performed to confirm a persistent versus a transient condition, taking the dynamics of the immune response. In foals, serum immunoglobulin concentration evaluation should take into account circulating levels of colostral antibodies, which varies individually (colostral IgG half-life 28-32 days). Serum IgM concentrations are more revealing of endogenous immunoglobulin production than IgG in foals less than 2 months of life.
Immunocompetent horses respond to infections with robust immunoglobulin production, whereas horses with humoral immunodeficiencies are hypogammaglobulinemic despite severe infection.
Immunological testing in horses:
- serum immunoglobulin concentrations (IgG and IgM; IgA not available currently);
- pre- and post-vaccination serum tetanus toxoid or pneumococcal antibody titers;
- peripheral blood lymphocyte phenotyping;
- peripheral blood lymphocyte proliferation;
- peripheral blood neutrophil function (oxidative burst activity).