Composition of the lymphoid cell populations from omental milky spots during the immune response in C57BL/Ka mice.

The lymphoid cell composition of milky spots was analyzed in unprimed mice before and after i.p. immunization with sheep red blood cells. Milky spots contained surface immunoglobulin-positive B lymphocytes, and T cells of the helper and cytotoxic phenotype. After secondary antigen challenge the number of lymphocytes increased up to 40-fold, B and T cells were found to segregate into distinct areas in situ, and lymphocytes were found to associate with I-A-negative stromal cells in vivo. These findings qualify milky spots as a peripheral lymphoid organ exhibiting a remarkable change in number and composition of lymphocytes in response to a local antigen stimulus.