Antigen can be classified on various basis.
On the basis of immune response antigen can be classified into:
- Haptens/ Incomplete antigens
- Immunogens/ Complete Antigens
Haptens/ Incomplete antigens:
- Antigens which are unable to generate the immune response themselves are termed as incomplete antigens. However on coupling with carrier proteins they can be immunogenic. They are also called haptens.
- When a molecule of haptens are coupled to carrier proteins they become accessible to immune system and function as an immunogen. They are usually non-protein substances and generally have low molecular weight (Less than 10,000 Daltons).
- g. Cardiolipin antigens, polysaccharide “C” of β-haemolytic streptococci, Capsular polysaccharide of pneumococcus etc.
Immunogens/ Complete antigens:
- The substance that induces specific immune response can be called as immunogen. Antigens which are able to generate immune response by themselves are known as complete antigens.
- These are usually proteinaceous in nature and generally have high molecular weight (more than 10,000 Daltons). They possess antigenic properties denovo. Some of them can be polysaccharide in chemical nature.
Carrier molecule is a non-antigenic component and helps in provoking the immune response. Example: Serum Protein such as Albumin or Globulin.
On basis of Source/Origin antigen can be classified as:
- Endogenous antigens
- Exogenous antigens
- Auto antigens
- These are antigens which originate from own body of host organisms. These are body’s own cells or sub fragments or compounds or the antigenic products that are produced. Endogenous antigens, by definition, are part of self, and the immune system is usually tolerant to them.
- The endogenous antigens are processed by the macrophages which are later accepted by the cytotoxic T – cells. E.g. Blood group antigens, HLA (Histocompatibility Leukocyte antigens) etc. Endogenous antigensare generated within normal cells as a result of normal cell metabolism, or because of viral or intracellular bacterial infection.
- Special type of endogenous antigens are those that distinguish one individual from another within the same species and are termed “alloantigens.”
- These are antigens which are foreign to host body hence are also called foreign antigens. These are antigens that enters the body of the organismfrom the outside, e.g. through inhalation, ingestion, or injection.
- These antigens enters the body or system and start circulating in the body fluids and trapped by the APCs (Antigen processing cells such as macrophages, dendritic cells etc.).
- The uptakes of these exogenous antigens by APCs are mainly mediated by the phagocytosis. E.g. Bacteria, Fungi, Viruses etc.
- These are usually a normal protein or protein complex (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease. These are not immunogenic under normal condition however due to genetic and environmental changes or factors immunological tolerance is lost and immune response is generated. E.g. Nucleoproteins, Nucleic acids, etc.
- These are antigens despite being a normal tissue constituent, is the target of a humoral or cell-mediated immune response, such as in autoimmune disease.g. Thyroglobulin, DNA, Corneal tissue, etc. These Are antigens to self-defense mechanism as they have been segregated from antigenic presentation during maturation of B and T- cells.
- Some antigens can be exogenous in origin but later can turn endogenous. E.g. Intracellular viruses
The response to self-antigens may have an important role in normal catabolic processes (i.e., antibodies to denatured IgG may help in eliminating antigen-antibody complexes from circulation; antibodies to oxidized low density lipoprotein (LDL) may help in eliminating a potentially toxic lipid).
The loss of tolerance to self-antigens, however, can also have pathogenic implications i.e. autoimmune diseases.
- Superantigens(SAgs) are a class of antigens that cause non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release. SAgs are produced by some pathogenic viruses and bacteria most likely as a defense mechanism against the immune system.
- When the immune system encounters a conventional T-dependent antigen, only a small fraction (1 in 104 -105) of the T cell population is able to recognize the antigen and become activated (monoclonal/oligoclonal response). However, there are some antigens which polyclonally activate a large fraction of the T cells (up to 25%). These antigens are called superantigens.
- They are a class of molecules that can interact with APCs and T lymphocytes in a nonspecific way. The superantigens act differently by interacting with MHC class II molecules of the APC and the Vb domain of the T-lymphocyte receptor. This interaction results in the activation of a larger number of T cells (10%) than conventional antigens (1%), leading to massive cytokine expression and immunomodulation.
- Examples include: Staphylococcal exfoliating toxins (scalded skin syndrome), Streptococcal pyrogenic exotoxins (shock), Staphylococcal enterotoxins (food poisoning) and Staphylococcal toxic shock toxin (toxic shock syndrome).
The diseases associated with exposure to superantigens are, in part, due to hyper activation of the immune system and subsequent release of biologically active cytokines by activated T cells.
- Theseare antigens found in different members of the same species. E.g. Red blood cell antigens A and B. Alloantigens elicit immune responses when cells or tissues of one individual are introduced into another.
- The alloantigens that elicit the strongest immune response are alleles of highly polymorphic systems, such as the erythrocytes (RBCs) of blood group antigens A, B, and O: some individuals carry the polysaccharide that defines the A specificity, others have B positive red cells, AB positive red cells, or red cells that do not express neither A nor B (O).
- Other alloantigenic systems that elicit strong immune responses are histocompatibility antigens of nucleated cells and tissues, the platelet antigens, and the Rh erythrocyte blood group antigens.
Examples of sensitization to exogenous alloantigens include:
- Women sensitized to fetal red cell antigens during pregnancy.
- Polytransfused patients who become sensitized against cellular alloantigens from the
- Organ transplants recipients who become sensitized against histocompatibility alloantigens, expressed in the transplanted organ.
- Theseare same or closely related antigens, sometimes present in tissues of different biological species, classes, or kingdoms. They are identical antigens found in the cells of different species. Examples: Forrssman antigen, Cross-reacting microbial antigens, etc.