Concentration of Salmonella pullorum Antigen and the Immunosuppressive Effect of Adrenocorticotropin in Growing Chickens

The role of antigen concentration on the immunosuppressive effect of adrenocorticotropin (ACTH) was tested in 6-week-old White Rock chickens that were immunized i.v. with 1 ml of heat-treated Salmonella pullorum at packed cell volume concentrations of .06 to .00015%, At 16 and 10 hr before the antigen (Ag) injection, the birds received either 4 IU . 100 g body wt-1 of ACTH i.m., or the gelatin (Gel) vehicle. Total, 2-mercaptoethanol sensitive (2-MES), and 2-mercaptoethanol resistant (2-MER) agglutinin antibody titers were determined. The ACTH significantly suppressed total agglutinin titers with the lower Ag concentrations (.0015 and .00015%) but not with the higher Ag concentrations. The ACTH significantly suppressed 2-MES titers only when Ag concentrations were low but suppressed 2-MER titers regardless of Ag concentration. The results indicated that there may be critical Ag concentrations capable of inducing maximal humoral antibody responses in moderate environments but which allow these responses to be suppressed by environments that stimulate increased pituitary-adrenal activity.