Dextran, of the variety commonly used as plasma expander, markedly altered antibody synthesis to an unrelated antigenic stimulus, SRBC, in two animal species, guinea pig and mouse. The time at which dextran was administered relative to antigen was found to be most critical for increasing or decreasing the number of IgM and IgG PFC. Furthermore, these times differed for the two species studied. Typically, when given to guinea pigs 6 h before SRBC, dextran caused a 20-fold rise in IgM-producing cells but had little effect upon IgG synthesis. However, if dextran preceded antigen by 24 h the same magnitude of increase was seen in IgG-forming cells while a decrease in IgM-producing cells occurred. In mice, a short 2 h interval between dextran and antigen favored cells synthesizing IgG and not those producing IgM. A longer 6 to 24 h lapse between dextran and antigen resulted again in an inverted pattern, i.e., an increase in IgM and a decrease in IgG-producing cells. In both species, if dextran was given 48 h before antigen, synthesis of IgG markedly decreased. At the cellular level dextran activated those B cells already in the vascular compartment. In stimulating the IgM response to SRBC, dextran appears either to substitute for T cells or to amplify the effects of that small number of T cells still present in bone marrow preparations. Dextran-altered mouse B cells synthesized SRBC-specific IgG in the presence of normal T cells at an earlier time than did normal B and T cells. However, dextran was unable to cause blastogenesis in vitro of guinea pig lymph node cells or mouse B, T, and spleen cells. The data suggest at least two effects that T cells exert upon B cells. One is to stimulate more B cells to produce IgM, a function accomplished by endotoxins, PWM, PPD, and the simple polysaccharide dextran. The other is to trigger shifts in synthesis of immunoglobulins M and G. Our observations are compatible with the view that a single cell is capable of synthesizing both of these immunoglobulins and that the stimulating factor for one may cause cessation of the other.

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