Immunological aspects of mammary involution.

Frequency of new intramammary infection is greatest during early involution, decreases during middle stages, and then increases prepartum. Penetrability of the teat canal, antibacterial properties of keratin, bacterial adherence, and epithelial sensitivity to toxins play a role in resistance. Leukocytes phagocytose bacteria and regulate expression of immune mechanisms, although their function is compromised during certain stages of involution. These cells increase to millions per milliliter as involution progresses and then decrease prepartum. Macrophages predominate in lacteal secretions, followed by lymphocytes and neutrophils. Lactoferrin, a major whey protein and iron chelator, is also associated with resistance to infection during the nonlactating period and may have immunomodulatory properties. Lacteal immunoglobulins increase throughout involution peaking prepartum and function by opsonizing bacteria, neutralizing toxins, and preventing bacterial adherence. Immunoglobulins are derived from blood or are produced locally by plasma cells present in the subepithelial mammary stroma. Plasma cells, lymphoid cells, and other protective leukocytes present in teat end tissues accumulate during infection and concentrations increase in response to local antigenic stimulation. Various aspects of the mammary immune system are compromised during periods of functional transition. Thus, vaccination, immunostimulation, accelerated involution, and intramammary devices are some methods now being tested to amplify local immunity and protect the gland from bacterial infection.