Antigens

Antigens

Foreign substances recognized by the immune system are referred to as antigens.
Requirements for immunogenicity
	Foreignness i.e. the degree of genetic difference between the antigen and the animal that encounters it.
	A large molecular weight - between 1000 and 6000 daltons immunogenicity is variable (insulin); molecular weights greater than 6000 daltons (tetanus toxoid) are generally immunogenic; molecular weights less than 1000 are non-immunogenic. Haptens are low molecular weight compounds such as drugs or antibiotics that are too small to cause an immune response unless they are coupled with much larger entities, such as proteins. The protein portion is called the carrier.
	Chemical complexity determines whether an immune response will be mounted to a molecule. Simple compounds, even if of high molecular weight, are not immunogenic. Thus:
		Virtually all proteins are immunogenic.
		Carbohydrates (polysaccharides) are potentially immunogenic. Polysaccharides that are part of complex molecules (like cell surface glycoproteins) elicit an immune response directed specifically against the polysaccharide moiety. An example is the ABO blood groups, which derive their antigenicity and immunological specificity from polysaccharides on the surface of the red blood cells.
		Lipids are usually not immunogenic but may become so if conjugated to a protein carrier. Lipids may therefore be regarded as haptens.
		Nucleic acids are poor immunogens by themselves but become immunogenic when conjugated to proteins.