Antigens

• Antigens are substances that can stimulate antibody generation and are recognized by an antibody molecule or T or B cell receptor, becoming the target of the immune response.
• Antigens can enter the body from the environment, such as inhaled macromolecules, ingested macromolecules, or molecules introduced beneath the skin.
• Antigens can also be generated within the cells of the body, such as proteins encoded by the genes of viruses that have infected a cell or aberrant proteins that are encoded by mutant genes.
• The main function of MHC class II is to bind to antigens derived from pathogens and display them on the cell surface for recognition by the appropriate T-cells.
• MHC class II is normally found only on antigen-presenting cells such as dendritic cells, mononuclear phagocytes, some endothelial cells, thymic epithelial cells, and B cells.
• The MHC class III region encodes several molecules important in inflammation, including complement components C2, C4, and factor B; tumor necrosis factor (TNF)-alpha; lymphotoxin; and three heat shock proteins.
• MHC class II binds peptides generated mainly from degradation of cytosolic proteins by the proteasome.
• MHC class II is found on the cell surface of all nucleated cells in the bodies of jawed vertebrates.
• Antigens presented by class II peptides are derived from extracellular proteins.
• Epitope, also known as antigen determinants, is the portion of antigen molecules which can be specifically recognized by antibody or antigenic receptor of lymphocytes.
• Paratope, also called an antigen-binding site, is a part of an antibody which recognizes and binds to an antigen.
• Viral antigens contribute to the pool of neoantigens for virus-associated tumors like cervical cancer and a subset of head and neck cancers.
• Neoantigens are those that are entirely absent from the normal human genome and are newly acquired and expressed antigens.
• Exogenous antigens are taken up by antigen-presenting cells like phagocytic cells like dendritic cells and macrophages, and B lymphocytes which are responsible for producing antibodies against the antigen.
• Incomplete antigens, also known as haptens, are unable to induce antibody formation on their own but can become immunogenic when covalently linked to proteins, called carrier proteins.
• Endogenous antigens are generated within previously normal cells as a result of normal cell metabolism or because of viral or intracellular bacterial infection.
• Chemical Nature of Immunogens: Complete antigens can induce antibody formation by themselves and can react specifically with these antibodies.
• Tumor antigens can appear on the surface of the tumor in the form of, for example, a mutated receptor, in which case they are recognized by B cells.
• Autoantigens 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 diseases.
• Immunogenecity is the ability of a particular substance, such as an antigen or epitope, to provoke an immune response in the body of a human or animal.
• Immunogen is a kind of stimulus that produces a humoral or cell-mediated immune response.
• Haptens are small molecules that elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself.
• Human Leukocyte Antigen (HLA) is a gene complex encoding the major histocompatibility complex (MHC) proteins in humans, and the proteins encoded by HLAs are those on the outer part of body cells that are (in effect) unique to that person.
• Superantigens are produced by bacteria and viruses, able to stimulate a large proportion of T lymphocytes via interaction with the variable domain of the β chain of the T cell antigen receptor (TCR-V β ), causing non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release.
• Epitopes formed by several adjacent amino acid residues are called linear determinants.
• Blood groups are created by antigens present on the surface of red blood cells.
• B cells epitopes bind antigen that is free in solution, and the epitopes they recognize tend to be highly accessible sites on the exposed surface of the immunogen.
• T-cell epitopes are peptides combined with MHC molecules, and there is no requirement for solution accessibility such as B-cell epitope.
• Tumor antigens are tumor associated transplantation antigens (TATA) that are expressed by tumor cells and some normal cells, and may be expressed at higher levels on tumor cells when compared to normal cells.
• The commonly used terms Rh factor, Rh positive and Rh negative refer to the D antigen only.
• The complexity of the Rh blood group Ags is due to the highly polymorphic genes that encode them.
• The immune system uses the HLAs to differentiate self cells and non-self cells, and any cell displaying that person's HLA type belongs to that person and, therefore, is not an invader.
• Tumor-specific transplantation antigens (TSTA) are unique to tumor cells and not expressed on normal cells, and are responsible for rejection of the tumor.
• Major Histocompatibility Complex (MHC) are cell surface proteins essential for the acquired immune system to recognize foreign molecules in vertebrates, which in turn determines histocompatibility.
• Recognition of antigens by T cells and B cells is fundamentally different.
• The Rh blood group system is the most complex system, consisting of 50 defined blood group antigens, among which the five antigens D, C, c, E, and e are the most important.
• Adjuvants are substances that, when mixed with antigens, enhance the antibody response to antigen itself.
• Alum, an aluminum potassium sulfate, precipitates the antigen, resulting in increased persistence of the antigen and increased phagocytosis.
• Incomplete Freund’s adjuvant, a mineral oil-based adjuvant, increases persistence of the antigen, stimulates mild granuloma.
• Complete Freund’s Adjuvant, a mineral oil-based adjuvant containing dead Mycobacterium, increases persistence of the antigen, stimulates a chronic inflammatory response (granuloma), and co-stimulatory signals.