Effects of antigen and antigen concentration on serum antibody titres in chickens primed per bursam

A novel method for intrabursal immunization was applied to study the priming effect in 2-day-old and 4-week-old chickens. This immunization method separates the bursa of Fabricius totally from the rest of the gut-associated lymphoid tissue (GALT), and closely mimics the natural way antigens come in contact with the bursal epithelium. Sheep red blood cells (SRBC) and Brucella abortus organisms in 4 different concentrations were used as antigens. In 4-week-old birds, a suppressive effect of per bursam priming on the secondary anti-SRBC titres was recorded, an observation not reported earlier. Priming had no effect on the secondary anti-SRBC titres in 2-day-old chickens. In both age groups, a positive priming effect on anti-Brucella titres was observed. Antigen concentrations too low to induce primary immune responses in serum were still capable of causing the priming effect. A certain minimum concentration, however, was required also for the priming effect to occur. The observed suppressive effect of priming on the secondary anti-SRBC titres suggests that bursa exerts inhibitory immuno-regulatory functions when handling certain antigens. Important factors are the age of the chickens and the antigen concentration used, which influence both the primary and the secondary immune responses.