<P> An immunogen is an antigen substance (or adduct) that is able to trigger a humoral (innate) or cell - mediated immune response . It first initiates an innate immune response, which then causes the activation of the adaptive immune response . An antigen binds the highly variable immunoreceptor products (B - cell receptor or T - cell receptor) once these have been generated . Immunogens are those antigens, termed immunogenic, capable of inducing an immune response . </P> <P> At the molecular level, an antigen can be characterized by its ability to bind to an antibody's variable Fab region . Different antibodies have the potential to discriminate among specific epitopes present on the antigen surface . A hapten is a small molecule that changes the structure of an antigenic epitope . In order to induce an immune response, it needs to be attached to a large carrier molecule such as a protein (a complex of peptides). Antigens are usually carried by proteins and polysaccharides, and less frequently, lipids . This includes parts (coats, capsules, cell walls, flagella, fimbriae, and toxins) of bacteria, viruses, and other microorganisms . Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides . Non-microbial non-self antigens can include pollen, egg white and proteins from transplanted tissues and organs or on the surface of transfused blood cells . Vaccines are examples of antigens in an immunogenic form, which are intentionally administered to a recipient to induce the memory function of adaptive immune system toward the antigens of the pathogen invading that recipient . </P> <P> Paul Ehrlich coined the term antibody (in German Antikörper) in his side - chain theory at the end of the 19th century . In 1899, Ladislas Deutsch (Laszlo Detre) (1874--1939) named the hypothetical substances halfway between bacterial constituents and antibodies "substances immunogenes ou antigenes" (antigenic or immunogenic substances). He originally believed those substances to be precursors of antibodies, just as zymogen is a precursor of an enzyme . But, by 1903, he understood that an antigen induces the production of immune bodies (antibodies) and wrote that the word antigen is a contraction of antisomatogen (Immunkörperbildner). The Oxford English Dictionary indicates that the logical construction should be "anti (body) - gen". </P> <Ul> <Li> Epitope--The distinct surface features of an antigen, its antigenic determinant . Antigenic molecules, normally "large" biological polymers, usually present surface features that can act as points of interaction for specific antibodies . Any such feature constitutes an epitope . Most antigens have the potential to be bound by multiple antibodies, each of which is specific to one of the antigen's epitopes . Using the "lock and key" metaphor, the antigen can be seen as a string of keys (epitopes) each of which matches a different lock (antibody). Different antibody idiotypes, each have distinctly formed complementarity determining regions . </Li> <Li> Allergen--A substance capable of causing an allergic reaction . The (detrimental) reaction may result after exposure via ingestion, inhalation, injection, or contact with skin . </Li> <Li> Superantigen--A class of antigens that cause non-specific activation of T - cells, resulting in polyclonal T cell activation and massive cytokine release . </Li> <Li> Tolerogen--A substance that invokes a specific immune non-responsiveness due to its molecular form . If its molecular form is changed, a tolerogen can become an immunogen . </Li> <Li> Immunoglobulin - binding protein--Proteins such as Protein A, protein G, and protein L that are capable of binding to antibodies at positions outside of the antigen - binding site . While antigens are the "target" of antibodies, immunoglobulin - binding proteins "attack" antibodies...</Li> <Li> T - dependent antigen--Antigens that require the assistance of T cells to induce the formation of specific antibodies . </Li> <Li> T - independent antigen--Polysaccharides (usually) that stimulate B cells directly . </Li> <Li> Immunodominant antigens--Antigens that dominate (over all others from a pathogen) in their ability to produce an immune response . T cell responses typically are directed against a relatively few immunodominant epitopes, although in some cases (e.g., infection with the malaria pathogen Plasmodium spp .) it is dispersed over a relatively large number of parasite antigens . </Li> </Ul>

The antigenicity of a molecule is due to