An Immunogen is a substance (or adduct) that is able to trigger a humoral and/or cell-mediated immune response.(Parham, Peter, 2009). It first initiates an innate immune response, which then causes the activation of the adaptive immune response.
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An antigen binds the highly variable immunoreceptor products (B-cell receptor or T-cell receptor)
once these have been generated. All immunogen molecules are also antigens, although the reverse is not true.
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. Antigens are usually proteins and polysaccharides, and less frequently, lipids.
This includes parts (coats, capsules, cell walls, flagella, fimbrae, 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 induce the memory function of adaptive immune system toward the antigens of the pathogen invading the recipient.
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