An illustration that shows how<unk> s induce the immune system response by interacting with an antibody that matches the molecular structure of an<unk>  In immunology, an<unk>  <unk>  is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of<unk> s in the body may trigger an immune response. The term<unk>  originally referred to a substance that is an antibody generator.<unk> s can be proteins, peptides (amino acid chains), polysaccharides (chains of monosaccharides/simple sugars), lipids, or nucleic acids.<unk> s are recognized by<unk>  receptors, including antibodies and T-cell receptors. Diverse<unk>  receptors are made by cells of the immune system so that each cell has a specificity for a single<unk> . Upon exposure to an<unk> , only the lymphocytes that recognize that<unk>  are activated and expanded, a process known as clonal selection. In most cases, an antibody can only react to and bind one specific<unk> ; in some instances, however, antibodies may cross-react and bind more than one<unk> . The<unk>  may originate from within the body ("self-protein") or from the external environment ("non-self"). The immune system identifies and attacks "non-self" external<unk> s and usually does not react to self-protein due to negative selection of T cells in the thymus and B cells in the bone marrow. Vaccines are examples of<unk> s in an immunogenic form, which are intentionally administered to a recipient to induce the memory function of the adaptive immune system towards<unk> s of the pathogen invading that recipient. The vaccine for seasonal influenza is a common example.
Antigen
Immunogen
Antibody
Specifier 
Allergen
Immunotoxin
Microantibody
Heterophile antibody