Cells Involved in the Immune Response

Abstract

Attempts were made to initiate the primary and secondary humoral immune responses to sheep red blood cells (SRBC) <i>in vitro </i>as determined by the hemolytic plaque-forming cell (PFC) response, with cell suspensions prepared from a variety of lymphoid organs of the rabbit – thymus, bone marrow, spleen, appendix, sacculus rotundus, Peyer’s patches, popliteal lymph node and circulating leukocytes. A number of different media and gaseous phases were utilized in order to establish the optimal conditions for the immune response <i>in vitro. </i>The induction of a secondary PFC response was consistently obtained with ‘memory’ spleen cells obtained from rabbits 3–6 months following intravenous immunization with SRBC but not with cells of any of the other lymphoid organs, and this response probably represents the activity of memory cells which reside in the rabbit spleen. A primary response was observed only with ‘normal’ spleen cells, and the medium which facilitated the response was different from that which facilitated the induction of the secondary response <i>in vitro. </i>It was also observed, using a medium in which normal spleen cells were incapable of generating PFCs, that mixed cultures of normal spleen and normal appendix or bone marrow cells could give a marked PFC response <i>in vitro.</i> Whether the PFC response to SRBCs obtained with the lymphoid cells of normal, unimmunized rabbits represents a true primary response, a secondary response, or a response of a different nature as a consequence of continuous subthreshold immunization of the rabbit with enteric microorganisms which cross-react with the antigen, remains to be determined. However, our initial successes with cultures consisting of cells of at least two distinct lymphoid organs in cases where the cells of any one of these organs could not respond, suggest that interaction of at least two functionally distinct cells is required and that the response observed <i>in vitro </i>is probably a primary immune response.