The primary antibody response elicited from mouse spleen explants by conjugates of the 3-nitro-5-iodo-4-hydroxyphenylacetic acid (NIP) hapten consisted mostly of the IgA class. Poly-L-lysine, pneumococcal polysaccharide Type SIII, keyhole limpet hemocyanin, and sheep erythrocytes were effective carriers in this system, whereas chicken globulin was not. The anti-NIP response against all of the immunogenic conjugates was detectable in culture media 4 days after explantation and immunization, and reached peak titers by 8–10 days. IgA was identified by sucrose gradient velocity centrifugation in conjunction with the use of a class-specific antiserum. The media collected at 4 days contained low titers of IgM antibody, whereas the peak response at 8 days consisted almost entirely of IgA.

The primary response IgA secreted by the spleen fragments was characterized as polymeric by its sedimentation rate through a sucrose gradient, and as polyvalent by its drastically greater avidity for NIP14BSA than for free NIP-aminocaproic acid. Its haptenated phage-inactivating activity was abolished by treatment with 0.1 M 2-mercaptoethanol.

These experiments indicate that precursor cells existing in the spleen before primary immunization can give rise to production of polymeric IgA.

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