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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.