The conjugate component of the vaccine, usually a protein, enhances the immunogenicity of the polysaccharide antigens derived from the bacterial capsule. Polysaccharides alone are poorly immunogenic in young children because they primarily induce a T-cell-independent immune response. Conjugating these polysaccharides to a protein carrier converts the immune response to a T-cell-dependent one, resulting in a more robust and long-lasting immunity. Histologically, this can be observed as increased activation and interaction between antigen-presenting cells (APCs) and T cells in the lymphoid tissues.
