immunogen

An immunogen is any substance that has the capacity to generate an adaptive immune response in a given host. Immunogens stimulate the production of antibodies by B cells and the activation of T cells, potentially leading to immunologic memory. By contrast, an antigen refers to any molecule or part of a molecule that can be bound by an antibody or a T cell receptor; not all antigens are immunogenic in every context.

Immunogenicity is influenced by factors such as foreignness to the host, molecular size and complexity, stability, and route of administration. Higher molecular weight, structural complexity, and repetitive epitopes generally promote stronger responses. Small molecules, or haptens, are poorly immunogenic unless conjugated to a larger carrier protein or peptide. Adjuvants and delivery systems, including alum or oil-in-water emulsions, can enhance immunogenicity by stimulating innate immunity or presenting antigens to lymphocytes more effectively. Genetic background, age, and immune status also shape the response.

Immunogens can be proteins, polysaccharides, lipids, nucleic acids, whole pathogens (live attenuated or inactivated), or synthetic constructs. They contain B cell epitopes that bind antibodies and T cell epitopes presented by MHC molecules. Epitopes may be linear or conformational. Examples used in research and vaccines include proteins such as keyhole limpet hemocyanin, toxins like diphtheria toxoid, and various bacterial polysaccharides.

Terminology notes: immunogenicity denotes the ability to provoke an immune response; antigenicity denotes the ability to be recognized by antibodies or T cell receptors. The relationship among immunogen, antigen, and epitope is central to vaccinology, immunotherapy, and diagnostic design.