Since placentas of pregnant sows are impermeable
to immunoglobulin passage, the neonates are born
without antibodies. Although immunocompetent,
piglets are unable to rapidly develop an active immune
response to protect them against respiratory diseases.
Their survival depends on the passive acquisition of
maternal immunity including at least three components:
(1) systemic humoral immunity, transmitted
through colostrum; (2) mucosal humoral immunity,
transmitted through milk; and (3) cellular immunity,
transmitted via maternal immunocompetent cells
present in mammary secretions.