The newborn piglet is exposed to a vast array of antigens from the moment it is born. Its immune system is naive and immature but competent to respond and produce an active immunity. The maternally derived immunity has to provide sufficient protection long enough while the piglet gradually develops its own active immunity. This is illustrated in Fig.3-4. In the wild a sow continues to suckle her offspring for several months. Weaning is gradual with plenty of time and opportunity for a wide range of antigenic stimulation. However, in modern pig production weaning is abrupt and at an unnaturally young age (e.g. 3 to 4 weeks. After weaning, the circulating humoral antibodies persist and continues to provide an effective protection against invasion of the pig's body. However at weaning, milk, the source of mucosal-associated antibodies is suddenly cut off. The antibodies that are present in the mucous decline within a day or so.