Quantitative ELISAs have been developed for the four immunoglobulin G (IgG) subclasses (IgGl, IgG2a, IgG2b and IgG2c) of the rat. These assays were shown to have excellent sensitivity, reproducibility and adequate specificity for studies of natural IgG and antigen-elicited IgG responses. The sensitivity (working range for each isotype assay) was: IgGl, 1.6 to 200 ng/ml; IgG2a, 8 to 200 ng/ml; IgG2b, 1.6 to 200 ng/ml; and IgG2c, 1.6 to 1000 ng/ml. The isotype sum and the direct assay of total plasma IgG agreed closely. The utility of these assays was demonstrated in a study of the development of total IgG and of the specific IgG response following immunization with tetanus toxoid (TT). For total IgG, the predominant isotype was lgG2b (64% of total IgG) followed by IgG2a (29%), IgGl (4%) and IgG2c (3%). In contrast, the anti-TT response was mainly of the IgGl subclass (57% of total anti-TT). These quantitative assays should prove useful for investigating the response to experimental vaccines and the influence of cytokines on class switching in vivo.
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