External secretions, including milk, contain large amounts of immunoglobulins (Ig) and innate factors of humoral immunity. The Ig isotype distribution, their origin, transport, and absorbability from the gut lumen differ greatly among various species. Milk of primates contains mainly secretory IgA (sIgA) which has unique biological properties. The functional advantages of sIgA compared with other Ig isotypes include its resistance to proteolysis, presence of 4 to 8 antigen-binding sites, carbohydrate moiety-mediated antiadherence properties, and inhibition of complement activation. Although IgA exhibits its protective effect mainly on mucosal surfaces, internalization by cells expressing various IgA receptors suggests additional protective functions.