Environment-genetic influences on immunocompetence.

The immunological responses of an animal are changing continually in response to perceived environmental changes. This is because the genetic background, the lifelong environment of animals and their interaction greatly influence immunological responsiveness. An animal's genetic background influences all factors related to immunocompetence. Among these are age of onset of immunocompetence, responsiveness to specific antigens, antibody titers, type of antibody and immune response, as well as the persistence of the responses. Defense by immunity must require considerable resources, because chickens with a high antibody response are smaller and have poorer feed efficiencies than those chickens whose antibody response is lower. An increase in the effectiveness of one defensive factor may result in reduced effectiveness of another factor. For example, chickens selected for a high antibody titer response to antigen have reduced effectiveness of macrophages. Environmental stresses influence the immune response. Stress at the time of the animal's contact with antigen results in a reduced antibody response. After the antibody response begins, stress has little effect. Stress promotes the sensitization of cell-mediated immunity but inhibits its effectiveness. A short-term stressor such as weaning is followed, in about 24 h, by a short period of reduced immunocompetence, even though lymphoid mass may be reduced. Reduced immunocompetence during stressful periods can be reduced by employing adrenal blocking chemicals, or by socializing animals to their handlers.