The developnent of delayed hypersensitivity as measured by footpad swelling follawing immunization with erythrocyte antigen closely parallels the development of the radioresistant, carrier erythrocyte specific thymus-derived cell population of the spleen capable of enhancing the anti-hapten response of a B cell population in vitro. These two thymus-derived cell activities have four properties in cammon. (1) The kinetics of sensitization are the same. (2) The dose of antigen required for sensitization is the same and is too small to elicit a humoral response. (3) The spcificity of the response with respect to the erythrocyte antigen and cross-reacting antigens is the same. (4) In neither case does the simultaneous injection of anti-erythrocyte antisera in amounts sufficient to suppress the humoral response to an immunogenic dose of antigen, effect the development of sensitization. The facts collectively indicate that the properties of the two cell populations, the one mediating footpad swelling and the other the helper effect, are very similar. This suggests the possibility that both activities may be mediated by a single mobile population of cells.
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