Development of cell-mediated immunity in young ruminants.

Development of cell-mediated immunity in young ruminants appears to be under the influence of the thymus. In sheep, prenatal removal of the thymus has little effect on postnatal growth of lambs. However, lambs are immunodeficient compared with normal controls, and they exhibit depressed delayed hypersensitive skin responses to antigens such as tuberculin purified protein derivative. Lymphopenia accompanying the immunodeficiency appears to be due to depletion of a particular population of T-cells (thymus derived) that have reduced response to mitogens and decreased numbers as the lamb matures. Young lambs are less responsive than adult sheep to vaccination with irradiated Trichostrongylus colubriformis larvae. The basis of this lowered responsiveness appears to be not only the immaturity of the cell-mediated immune response but also the segregation of the lambs into high and low responders. This immune responsiveness is possibly under the genetic control of the ovine major histocompatibility complex. It may be possible to select and breed sheep and cattle for responsiveness to vaccination against parasitic, viral, and bacterial diseases.