Antigens are high - molecular substances of organic origin, capable when introduced into the body to cause the formation of specific proteins - antibodies. Antigens are able to connect with those antibodies that have arisen under their influence. Antigenic properties of proteins and polysaccharides (the so-called full-antigens). Antigens are widely used in the diagnosis of infectious diseases to determine the nature of antibodies. The antigens used for this purpose, called diagnosticums.
In addition to full antigens can cause the formation of antibodies and respond to them, there are defective - haptens (mostly carbohydrates and lipids), which responds with the appropriate antibodies, but will not cause their education. The haptens are compounds that give the antigens certain specificity. The haptens turn into a full-antigens addition of protein. Some inorganic compounds, including pharmaceuticals (antipyrine, quinine, iodine and others), when the body can connect to its proteins and buy antigenic properties. The nature of immunological reaction antigens can be designated as agglutinogen that cause the formation of antibodies that govern the response of agglutination (see); precipitance - antigens, causing the formation of antibodies, participating in the reaction precipitation (see).

Antigens (from the Greek. anti - against and gennao - create, produce) are chemical compounds that, when injected into a human or animal body cause the formation of antibodies. Antigenic properties has a wide range of natural high-molecular compounds and first of all proteins, polysaccharides (see Vi-antigen) and their complexes. In addition, antigens can be artificially synthesized proteins and protein complexes with the most diverse in the structure of chemical compounds. When introduced into the body of conjugated compounds formed antibodies (see), specifically responsive to put into protein simple chemical group, which, when introduced into the body without the media, does not cause the formation of antibodies. These compounds because of their immunological passivity was named incomplete antigens, or haptens.
Currently, not all else is known about the conditions of antigenicity of a substance, however, no doubt that the degree of antigenicity protein is determined by some specific features of their chemical structure, which should be attributed a relatively high molecular weight. Indeed, relatively simple in composition and structure of Protamine and gelatin are not antigens, and egg and serum albumin (mol. weight 40 000-70 000) immunogenic in less than gamma globulins (mol. weight 160 000) or hemocyanin (mol. weight of 300 000 and more). An indispensable condition of antigenicity is the difference between the structure of the substance of any of the substances present in the body of the recipient. Own body proteins are not the antigens, if they have not been subjected to chemical processing, capable of changing their structure. In connection with the genetic differences between individuals of the same species of similar proteins (such as gamma globulin) they may have some differences in the structure. In this regard, one animal protein of this type can be antigen for another animal of the same species. Such antigens are called isoantigen.
In certain pathological conditions own proteins, polysaccharides and their complexes as a result of changes in the chemical structure acquire the ability to autoimmunization. These proteins known when acquired hemolytic anemia, idiopathic thrombocytopenic purpura, nodular the nodosa, lupus erythematous and other diseases.
In connection with complex chemical structure of natural proteins and polysaccharides on the surface of their macromolecules is a significant number of antigenic determinants of different structures. Antigenic heterogeneity of individual proteins and polysaccharides results for immunization to the formation of a large set of antibodies of different specificity (see Immunity). If some proteins or polysaccharides of different origin individual antigenic determinants similar in its structure, formed antibodies will cross-react with both antigens. Antigenic relationship along with specific differences are established for similar proteins from different species (serum albumin, gamma-globulin) or somatic 0-antigens of coliform. In some cases the same antigenic determinants are found in very different in origin substances, for example of group-specific And antigens of eritrotsitov person and capsular polysaccharide pneumococcal type XIV. Serologically related cellular antigens are far from each other species was named heterogeneous A. Examples of such A. serve as antigens Forssmann - substances which when injected rabbits cause formation of mutton hemolysins.
Natural A. can meet in particulate and soluble form. Since the first investigated with immune serum agglutination reaction, they are called agglutinogenic. Accordingly soluble antigens, analyzed in the reaction of precipitation, sometimes called precipitiously. In corpuscular A. bacterial origin distinguish A. actually the body's cells and flagellar A., representing the chemical structure of proteins. If there are identical in structure of the determinants of agglutinogen and precipitance can have the same specificity of serological. Despite serological kinship, immunogenic activity of agglutinogenic and precipition differs: the formation of antibodies to agglutinogen is usually much harder.