Isotype cross-reactivity of anti-porcine IgG and IgA reagents

Reliable assessment of vaccine induced antibody responses depends on the specificity and validation of immunological reagents, however, many commercially available antibodies remain insufficiently validated and may exhibit substantial off target binding. This limitation is particularly relevant for isotype specific analyses, where cross reactivity can profoundly affect data interpretation. In this study, we examined serum IgA responses following vaccination in pigs using two different commercially available polyclonal anti-porcine IgA antibodies and evaluated their specificity against monoclonal antibody-based assays and recombinant immunoglobulin isotypes. Initial analyses using a widely applied polyclonal antibody showed IgA response kinetics closely resembling IgG responses, though at lower titers, consistent with numerous previous reports. The unexpectedly similar half-lives of serum IgA and IgG prompted us to test an alternative polyclonal antibody, which revealed only transient IgA responses detectable one week after immunization. This short-lived IgA response was independently confirmed using a biotinylated monoclonal antibody ELISA specific for porcine IgA. Correlation analyses across all time points, together with direct binding assays employing isotype-switched monoclonal antibodies and recombinantly produced porcine immunoglobin isotypes, demonstrated pronounced cross reactivity between one commercial anti-IgA polyclonal antibody and porcine IgG, and a cross reactivity of both commercial anti-IgG polyclonal antibodies, which showed comparable levels of detection of IgA as the IgA-specific polyclonal antibodies. Collectively, these findings demonstrate a critical lack of specificity in commonly used polyclonal secondary antibodies for porcine IgA and IgG detection. Importantly, antibody choice fundamentally alters the interpretation of antibody isotype responses, highlighting the need for rigorous validation of immunological reagents in both preclinical vaccine research and infectious disease immunopathology studies.