A possible role for the lymphocyte in the mechanism of eosinopoiesis has been examined.

Procedures known to deplete or inactivate the pool of recirculating lymphocytes such as neonatal thymectomy, administration of antilymphocyte serum, and prolonged thoracic duct drainage, either singly or in combination, resulted in a highly significant reduction in the eosinophil response to trichinosis.

Irradiated animals exposed to parasitic challenge did not develop eosinophilia unless reconstituted with lymphocytes as well as bone marrow cells. When "memory" cells were used instead of normal lymphocytes, a "secondary" type of eosinophil response was observed.

Transfer of a primary eosinophilia was achieved adoptively with a population of living large lymphocytes from thoracic duct lymph and peripheral blood, but not with blood plasma or cell-free lymph. The potency of the active lymphocytes was not impaired by enclosing them in cell-tight diffusion chambers, indicating that they exerted an effect on bone marrow by agency of a diffusible factor.

The demonstration of a role for lymphocytes in induction of the eosinophil response to this kind of stimulus supports the conclusion that eosinophilia belongs in the category of immunologic phenomena.

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