Ig class- and IgA subclass-specific immune responses to protein and polysaccharide Ag were studied in serum, external secretions, and at the single cell level in peripheral blood of systematically immunized adults. Immunization with tetanus toxoid induced predominantly IgG antibody responses in serum and in the PBMC. The IgA antibody response was low, and was mostly of the IgA1 subclass. In contrast, immunization with polysaccharide Ag (Haemophilus influenzae type b, Neisseria meningitidis serogoup A, C, Y, W-135, and Streptococcus pneumoniae capsular polysaccharides) elicited a major IgA response predominantly of the IgA2 isotype. Analysis of the molecular forms of secreted IgA antibodies indicated that polymers were produced early after immunization, irrespective of the nature of the Ag. When compared with serum antibody and to PBMC cell responses, systemic immunization with polysaccharides induced a minor salivary response dominated by IgG and IgM antibodies. In contrast, the presence of antipolysaccharide antibodies in bile, irrespective of their isotype, paralleled the serum response 14 days after the immunization with polysaccharide Ag. These results suggest that biliary Ig were mostly derived from serum. Different patterns of the expression of MHC class II Ag on T cells, B cells, and monocytes during the course of immunization with protein or polysaccharide Ag were observed: whereas protein Ag induced a high frequency of HLA-DP- and HLA-DR-expressing cells early in the course of immunization, polysaccharide vaccines elicited low and protracted increases of HLA-DP+ T cells. Polysaccharide vaccine covalently coupled to a protein carrier induced a higher frequency of antipolysaccharide antibody-secreting cells in peripheral blood and increased the IgG to IgA ratio among polysaccharide-specific antibody-secreting cells.