The periellipsoidal white pulp of the avian spleen was studied in chickens and guinea fowl. A novel monoclonal antibody, designated NIC2, recognizes bursal secretory dendritic cells but not bursal and germinal center B-cells. The NIC2 mAb heavily stains the PWP cells, suggesting that these cells are not B lymphocytes. The number of NIC2 positive cells increases after hatching and by 12 weeks of age form "compact units" into which the ellipsoids are embedded. The carrageenan injection resulted in severe cell depletion in the PWP and rest of the PWP cells are loaded with carrageenan indicating their phagocytic capability. While bursectomy results in a moderate cell depletion, the major change is the appearance of an ill-defined cell population. Among these, cells some of them degenerate, suggesting that in bursaless chicken there is a histological and functional maturation inhibition, even cell death. The PW is an antigen-rich environment, which consists of an immature cell population that is capable of differentiation toward phagocytic cells. The close topographical relationship between the PWP and the ellipsoid supports our hypothesis.
