Immunoglobulin and peripheral B‐lymphocyte concentrations in Fell pony foal syndrome

Summary Reasons for performing study : Fell pony foals are affected by a congenital fatal disease that leads to profound anaemia and immunodeficiency. Previous studies comparing healthy and affected foals have shown normal T‐cell populations, but a severe B‐lymphopenia. Objectives : To measure the levels of individual immunoglobulin subisotypes in normal and affected Fell ponies and correlate these levels with the number of peripheral B‐lymphocytes. Methods : Serum levels of individual immunoglobulin subisotypes were measured by ELISA and correlated with the number of peripheral B‐lymphocytes (measured by flow cytometry). Results : Affected foals had significantly reduced serum levels of IgM, IgGa, IgGb and IgG(T) that coincided with the normal reduction in maternally derived immunoglobulin in foals and, with the exception of IgGb, correlated strongly with the B‐lymphopenia. Conclusions : These data suggest that affected foals are unable to produce their own immunoglobulins. Therefore, once maternal immunity has waned, it may be the lack of specific foal‐derived immunoglobulin that gives rise to the clinical signs of immunodeficiency. Potential relevance : Measurement of IgM after age 4 weeks may provide a more accessible means of confirming the status of future affected Fell pony foals than the measurement of B‐lymphocytes (a technique limited to a few specialist laboratories).