In vivo response to vaccination
When persistently low serum immunoglobulin concentrations are measured, an in vivo vaccination response test may be necessary to confirm the impaired antibody production. Immunocompetent horses should respond to vaccines despite the stress of infections. In foals, the use of this test may be limited because of age-dependent developmental factors in the humoral response to vaccines.
Two types of vaccines can be used to test the humoral response:
- tetanus toxoid - a protein vaccine that uses T cell-dependent humoral response (i.e. it requires B and T cell interaction in the lymphoid tissues before B cells become plasma cells, and secret antibodies);
- pneumococcal - a polysaccharide vaccine that uses T cell-independent humoral response (i.e. it does not require B and T cell interaction for antibody production)
Using ELISA, serum pre- and post-vaccination samples (15-21 days apart) can be tested for antibody titers against the protein or sugar vaccine antigens. The interpretation of results should include the clinical history, serum immunoglobulin concentrations, and peripheral blood lymphocyte phenotyping.
Immunological testing in horses:
- serum immunoglobulin concentrations (IgG, IgM, IgA);
- 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).