Synthesis and secretion of Ig by chicken lymphoid cells was studied. Both spleen and bursa cells synthesize and secrete IgM and IgG whereas Ig was not detected in thymus cells. In contrast to the spleen cells which synthesize H and L chains in balanced quantities, the bursa cells synthesize and secrete free L chains. In addition to the lymphoid cells which secrete IgM or IgG, the bursa appears to contain a cell population which synthesizes nonsecretory Ig. The structure of this Ig was studied by specific serological precipitation and by SDS-acrylamide gel electrophoresis. The H chains of this nonsecretory Ig are serologically related to µ-chains and exhibit a smaller molecular weight (i.e., ∼50,000) in SDS-acrylamide gel electrophoresis than H chains of IgG and IgM synthesized by the spleen cells (i.e., ∼70,000).

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