Passive Immunity in the Foal: Measurement of Immunoglobulin Classes and Specific Antibody

SUMMARY Concentrations of immunoglobulins (IgG, IgA, IgM, IgG(T), and aggregating immunoglobulin) were similar in presuckle colostrum and serum of the mare. Colostral immunoglobulins decreased rapidly after the foal was permitted to suckle, and by 16 days, IgA was the predominant immunoglobulin of milk. Serums from 4 foals before suckling contained IgM, and serum from 1 of these 4 foals had a detectable amount of IgG. All of the 5 immunoglobulins measured were absorbed by the foal, producing concentrations similar to those in the mare's serum within 24 hours after the foal was born and permitted to suckle. Afterwards, foal serum immunoglobulin concentrations began to decrease, with minimums of IgG and IgG(T) occurring 30 to 60 days after the foal was born. Probable synthesis of IgG, IgG(T), and IgA began in the foal within 24 days after birth, and synthesis of IgM probably began before birth. Titers of specific antibodies to viral antigens in the mare and in the foal were similar within 24 hours after the foal was born. These antibodies were not detectable by 1 month unless antigenic stimulation occurred.