B cell differentiation in the bursa of Fabricius and spleen of embryos and chicks immediately after hatching

Flow cytometric analysis and immunohistochemical observation were used to qualitatively and quantitatively clarify the nature of B cell differentiation in the bursa of Fabricius of chick embryos and to determine the timing of antibody class switching in chicken spleens based on positivity of IgM and IgG on and in the cells. In the bursa, the sIgM‐positive cell population formed from the 12^th to 15^th day of embryogenesis. The proportion of sIgM‐high expressing (sIgM^high) cells was lower among bursacytes than splenocytes of hatched chicks, suggesting that the sIgM^high bursacytes are to be released to peripheral sites. The proportion of sIgM^high cells was higher at 0 days old than at any other examined stage of development. Colonization of the spleen by B cells occurred between the 18^th day of embryogenesis and 0 days old. Antibody class switching was thought to start in the spleen between 1 and 2 weeks of age, because IgG‐positive cells were present in the spleen of 2‐week‐old chicks, but not 0‐day‐old or 1‐week‐old chicks.