The new born animal is poor at making active immune responses because it has too few peripheral dendritic cells and lymphocytes. Maternal IgG is the means of giving temporary immunity against environmental pathogens. First infection then occurs when the animals has some residual maternal IgG but are old enough to make a good immune response.

Foals can be checked by ELISA to insure they have ingested enough cholostral IgG ; if not, or if they are weakened by infection, then immune plasma from horses negatives for A/Q blood-groups can be given to boost their maternal immunity (See Sarah Stoneham,Collection and administration of plasma to a newborn foal. In practice,July 1997, pp384-5.)

Maternal antibody can also prevent live vaccines replicating and block inactivated antigen. If vaccination is too early maternal antibody prevents an active immune response and if it is too late disease may occur before vaccination.