A conjugate vaccine is created by covalently attaching a poor antigen to a strong antigen thereby eliciting a stronger immunological response to the poor antigen. Most commonly, the poor antigen is a polysaccharide that is attached to strong protein antigen. However peptide/protein and protein/protein conjugates have also been developed.
B cell response to a capsular polysaccharide is T cell independent, meaning that B cells can produce antibodies without T cell stimulation. By conjugating the polysaccharide to a protein carrier, a T cell response can be induced. Normally, polysaccharides by themselves cannot be loaded onto the MHC complex of antigen presenting cells (APC) because MHC can only bind peptides. In the case of a conjugate vaccine, the carrier peptide linked to the polysaccharide target antigen is able to be presented on the MHC molecule and the T cell can be activated. T cells stimulate a more vigorous immune response and also promote a more rapid and long-lasting immunologic memory. This technique for the creation of an effective immunogen is most often applied to bacterial polysaccharides for the prevention of invasive bacterial disease.