The invention provides a heterogeneous immunoassay for detection of antibodies and antigens based on specific antigen-antibody immune complex formation with multiple antigen-bearing conjugate components. The invention further provides means for optimizing the assay format for the detection of both low and high-affinity antibodies, and provides means for quantitative detection of both antibody and the corresponding antigen present in a sample. In preferred embodiments, the invention can be practiced to detect presence in biological fluid samples of antibodies to Bacillus anthracis related antigens.