Antigens are defined as substances capable of stimulating the production of antibodies, with which they react specifically. This definition is incomplete since it is well known that antigens can induce cell-mediated immune responses. So antigens are all molecules introduced into the body that can induce an immune response; that is to say the induction of the production of specific immune effectors (humoral or cellular) and to react with these.
antigen is a category of molecules (a molecular species) defined by its antigenic specificity. And, it defines antigenic specificity as the property of a given antigen to combine with a given (usually heterogeneous) population of antibodies. A given antigen can combine with several different or identical antibodies.
According to origin
– Xeno-antigen XENO ANTIGENS: these are the antigens present in all individuals of one or more species distinct from that to which the immunized subject belongs.
– Allo antigens (Iso antigens): these are antigens found in a group of individuals of the same species and can induce an immune response in individuals who do not have them.
– Auto antigens: these are the antigens of an individual that can induce an anti-self immune response.
Depending on whether the production of antibodies depends on T cells or not
– Thymo-dependent antigens: are those against which the production of antibodies requires the help of T lymphocytes. This category is mainly represented by proteins.
– Thymo independent antigens: are those against which the production of antibodies does not require the help of T lymphocytes; such as polysaccharides and lipopolysacharides which are characterized by the presence of repetitive epitopes.
According to the chemical nature
Protein antigens: these are the most immunogenic antigens. There are several types:
– Natural proteins: these are the main constituents of living things. They are encoded by corresponding genes.
– Artificial proteins: these are proteins whose natural core is on which side sequences are grafted.
– Synthetic proteins.
Polysaccharide antigens: the immunogenic power of these antigens is weak. The simple polysaccharides have repetitive antigenic determinants which can activate B lymphocytes without resorting to T lymphocytes. The antigenic determinants of these antigens are sequential and nonconformational determinants, of approximately 6 sugars.
Lipid antigens: lipids are not immunogenic. however, after their association with proteins, they can induce an immune response. They play the role of haptens. In certain diseases we find anti phospholipid antibodies.
Nucleic acids: the immunogenicity of these substances is controversial when all attempts at immunization fail. However, immunization with nucleic acids associated with proteins induces the production of specific antibodies.
According to physical properties (solubility)
Soluble antigens: constitute the majority of antigens in nature (proteins, polysaccharides, etc.)
Particulate antigens: correspond to all particles, living or inert, which can induce an immune response (bacteria, virus, cell, parasite, etc.)
- in vitro diagnosis
- skin tests