The effect of neonatally initiated injections of anti-µ or anti-δ antibodies on IgM, IgD, IgA, IgE, IgG1, IgG2a, IgG2b, and IgG2c immunoglobulin subclasses are described. As in chickens and mice, anti-µ suppression was demonstrated to be effective on rat IgM, IgA, and IgG classes but also on IgD and IgE classes and on the four subclasses of IgG. The anti-δ suppression clearly affected the IgD synthesis but also that of IgE. The different IgG subclasses were more or less diminished, but, on the contrary, the IgG2c serum level from day 55 on was largely increased. The IgM and IgA synthesis both were slightly affected with respect to their serum levels but significantly at the membrane level.

Rats suppressed by anti-µ or anti-δ sera were unable to build up a reaginic immune response taken as an example of T-dependent immune response.

The anti-µ suppression on all the immunoglobulin classes confirms that the IgM receptors are the first to appear on the precursors of the B lymphoid cells. The IgD suppression also confirms that the majority of B lymphoid cells carry IgD receptors during their maturation. Moreover, the absence of reaginic immune response indicates that the lack of the IgD receptor can make animals unable to produce primary IgE immune response.

The IgD receptor may be considered as an unspecific isotype-positive signal necessary for the majority of the B lymphoid cells to react for the first time to a specific antigen stimulation. The differential results of the anti-δ suppression, contrary to the general suppression obtained by the anti-µ inoculation, could be explained by the more or less important requirement by each isotype cell line, to be stimulated primarily by an antigen with the help of IgD class receptors.

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