Proteins function as the most potent immunogens, with polysaccharides ranking second. In contrast, lipids and nucleic acids generally do not serve as immunogens unless they are complexed to proteins or polysaccarides. Large insoluble macromolecules generally are more immunogenic than small, soluble ones because the larger molecules are more readily phagocytosed and processed. For example, it is thought that because of the small size of short peptides, peptides rarely encompass sufficeint T cell epitopes in order to provide the necessary cytokine help in the induction of strong immune responses. One strategy used to overcome this problem of immunogenicity include the linking of the peptide to large highly immunogenic protein carriers. The carrier proteins contan large number of peptidic T cell epitopes which are capable of being loaded into MHC molecules, thereby providing bystander T cell help, and/or alternatively the use of strong adjuvants in the vaccine formulation. Examples of these highly immunogenic carriers include the Diptheira and Tetanus toxodis (DT and TT), Keyhold limpet Haemocyanin (KLH) and the pruified protein derivative of Tuberculin (PPD).
Common Antigenic Carrier Proteins
Keyhole limpet hemocyanin (KLH): is the best known carrier protein that is coupled to a synthetic peptide antigen in order to elicit a stronger antibody response. KLH is prone to aggregation which is an important feature that makes KLH a very strong carrier immunogen for commercial antibody production. While KLH is a very good immunogen, it possesses limited solubility and is prone to spontaneous aggregation and precipitation, making it difficult to work with.