Direct stimulation of lymphoid tissue of the chicken. II. Changes in lymphoid organs following intrathymic and intravenous injection of antigen

Lymphoid tissue from chickens immunized intrathymically or intravenously with BSA was examined histologically at various time intervals after challenge. Quantitative analyses of the thymus medulla and the periarterial area of the spleen were based on 4080 to 6942 counted cells. Three days after immunization, thymic lobes injected with BSA exhibited an increase in the number of blasts. This was followed by a rise in plasma cell population up to 5 days. Significant diminution of small lymphocytes and moderate increase of medium and large lymphocytes occurred after immunization. Contralateral noninjected thymic lobes of the same chickens showed similar cellular kinetics. In thymuses of birds immunized intravenously with BSA there was a moderate increase in plasma cells. Thymic lobes of intrathymically or intravenously immunized bursectomized chickens were normal. In the spleen, an augmented number of blasts was found 3 days after intrathymic or intravenous challenge. Intrathymic injection of BSA produced profound changes in the bursal follicles and intense plasmacytic reaction 3 and 9 days after immunization. The results are discussed in the light of the hypothesis that the chicken thymus is not only a primary lymphoid organ, but also an organ capable of acting as a secondary lymphoid tissue.