This thesis contains a review of the immune components in porcine mammary secretions. The review includes a discussion of the mammary gland immune system including the specific immune cell types found, and their location, within the mammary gland. This is followed by an overview of the source, quantity and function of immunoglobulins found in swine mammary secretions. The cellular components of milk and colostrum are reviewed, and their possible role in the active immune response of the neonate is discussed. Additionally the presence of non-specific immune components in mammary secretions and their functions are reviewed. Specific research exploring the potential shedding of porcine reproductive and respiratory syndrome virus (PRRSV) in mammary secretions is presented. In this study naà ¯ve gilts challenged with PRRSV late in gestation shed virus in their mammary secretions. PRRSV was demonstrated both in the cellular portion of their secretions as well as the whey. Antibody in the whey did not appear to prevent shedding of the virus. These same animals challenged in a subsequent gestation did not shed PRRSV in their mammary secretions. Additionally, sows in commercial herds that were either vaccinated or previously exposed to PRRSV, did not shed PRRS virus in their milk.