What happens in an antigen antibody reaction?
Antigen-antibody reaction
antigen is a molecular structure, which causes stimulation of immune system .This structure such as cell wall , cell membrane, toxins or other part of microorganisms which called pathogens .
The antigen may be from inside the body ("self-antigen") or from outside environment ("non-self"). The immune system can recognize and attacks external antigens and usually does not react to antigens from inside the body due to negative selection of T cells which present in the thymus except in special cases.
we can classify antigen into:
Exogenous antigens
Exogenous antigens are come from out side the body such as pathogens which enter by respiratory system, digestive system or by injection . The immune system's response to these antigens is usually subclinical. By endocytosis or phagocytosis, exogenous antigens are engulfed by the antigen-presenting cells (APCs) and divided into fragments. APCs then present the fragments in their surface to T helper cells (CD4+) using class II histocompatibility molecules on their surface. Some T cells are specific for the peptide:MHC complex.Then, they are activated and begin to secrete cytokines, substances which activate cytotoxic T lymphocytes (CTL), AB-secreting B cells, macrophages and other cells .
Some antigens start out as extracellular pathogen and later become intracellular pathogen (for example, intracellular viruses).
Endogenous antigens
Endogenous antigens are started from normal cells as a result of cell metabolism, or because of viral or intracellular bacterial infection. The antigen fragments are then presented on the surface of the cell in the complex with MHC class I molecules. If activated cytotoxic CD8+ T cells recognize them, the T cells secrete different toxins which cause the infected cell lysis or apoptosis .
Autoantigens
An autoantigen is often either a normal protein or protein complex (and sometimes DNA or RNA) which is detected by the immune system of patients suffering from a specific autoimmune disease. , These antigens usually should not be the target of the immune system, but in case of autoimmune diseases, their associated T cells are not deleted and instead attack.
Neoantigens
Neoantigens are those which are entirely absent from the normal human genome. These are of relevance to tumor control. They can be directly detected and quantified through a method called MANA-SRM which developed by a molecular diagnostics company.
Tumor antigens
They are antigens that are presented by MHC class I or MHC class II molecules on the tumor cells surface. These antigen which found only on such cells are called tumor-specific antigens (TSAs) and usually result from a tumor-specific mutation. The most common are antigens that are presented by tumor cells and normal cells, called tumor-associated antigens (TAAs). Cytotoxic T lymphocytes which detect these antigens may be able to destroy tumor cells.
Antibody
An antibody (Ab), also called an immunoglobulin (Ig),is a large, Y-shaped protein secreted mainly by plasma cells the immune system use these antibody to neutralize pathogens or antigen. Each edge of the "Y" of an antibody contains a paratope (analogous to a lock) which is specific for one type of particular epitope (analogous to a key) on an antigen. these epitopes allow the two structures to bind together with precision.
Using this binding mechanism, an antibody can tag a microorganism or an infected cell for attack by other parts of the immune system.Also it can neutralize its target directly (by inhibiting a part of a microbe which is essential for its survival and invasion ). Depending on the antigen, the binding can be impede the process causing the disease or may activate macrophages to destroy the foreign antigen. The antibody can communicate with the other components of the immune system by its Fc region (located at the base of the "Y"). It contains a conserved glycosylation site involved in these interactions. The antibody production is the main function of the humoral immune system.
Antibodies types
Antibodies can come in different types called isotypes or classes. There are five antibody isotypes In placental mammals known as IgA, IgD, IgE, IgG, and IgM. the "Ig" prefix stands for immunoglobulin (a name sometimes used interchangeably with antibody) .They differ in their functional locations , biological properties, and ability to deal with different antigens. The different suffixes of the antibody isotypes resemble to the different types of heavy chains which the antibody contains.in each heavy chain class named alphabetically: α (alpha), γ (gamma), δ (delta), ε (epsilon), and μ (mu). This gives names to IgA, IgG, IgD, IgE, and IgM, respectively.
Immunoglobulin A (IgA or sIgA in its secretory form)
plays an important role in the immune function of mucous membranes. The amount of IgA which produced in mucosal membranes is more than all other types of antibody combined. In absolute terms, the intestinal lumen ,there are between three and five grams are secreted each day. This represents up to 15% of total immunoglobulins produced in the body.
IgA consists of two subclasses (IgA1 and IgA2) .it can be produced as a monomeric as well as a dimeric form. The IgA dimeric form is more prevalent and is also named secretory IgA (sIgA). sIgA is the main immunoglobulin presented in mucous secretions, including tears, saliva, sweat, colostrum and body secretions from the genitourinary tract, gastrointestinal tract, prostate and respiratory epithelium. It is also present in small amounts in blood. It protects the other immunoglobulin from being degraded by proteolytic enzymes; so, sIgA can survive in the harsh gastrointestinal tract environment . It also can provide protection against microbes which multiply in body secretions.sIgA can inhibit inflammatory response of other immunoglobulins.
Immunoglobulin D (IgD)
represent about 1% of proteins in the plasma membranes of immature B-lymphocytes .It is often co-expressed with another type of antibody called IgM. IgD is also present in a secreted form that is found in very small amounts in blood serum, which represent up to 0.25% of immunoglobulins in serum. Secreted IgD is present as a monomeric antibody and has two heavy chains of the delta (δ) class, and two Ig light chains.
Immunoglobulin E (IgE)
it has only been found in mammals. IgE is secreted by plasma cells. Monomers of IgE composed of two heavy chains (ε chain) and two light chains, with the ε chain which containing 4 Ig-like constant domains (Cε1-Cε4). IgE's main function is immunity to parasites like helminths such as Schistosoma mansoni,and Trichinella spiralis. IgE is used during defence of immune system against certain protozoan parasites like Plasmodium falciparum.IgE may have evolved as a last line of defense to protect against venoms.
IgE also plays an essential role in type I hypersensitivity, which causes various allergic diseases, like allergic asthma, most types of sinusitis, food allergies, and some types of chronic urticaria and atopic dermatitis. IgE also has a vital role in responses to allergens, such as: anaphylactic reactions to drugs, bee stings, and antigen preparations used in desensitization immunotherapy.
Immunoglobulin G (IgG)
it has four forms. It provides the majority of antibody-based immunity against invading pathogens.Also, it is the only antibody which capable of crossing the placenta to give passive immunity to the fetus.
Antigen-antibody interaction
antigen-antibody reaction is a special chemical reaction between antibodies and antigens during immune reaction. The antigens and antibodies combine by a reaction called agglutination. It is the main reaction in the body this process protects the body from complex foreign molecules, like pathogens and antigens. In the blood, the antigens are specifically and with more affinity combine by antibodies to make an antigen-antibody complex. then it is transported to cellular systems . the cellular system can destroy or deactivate this complex.
Application
Antigen-antibody interaction is usually used in laboratory techniques for serological test of blood compatibility and also various pathogenic infections. The most using is in ABO blood group determination, that is helpful for blood transfusion. it also uses in applications include ELISA,immunofluorescence, and immunoelectrophoresis.
Precipitation reaction
Soluble antigens interact with soluble antibodies in presence of an electrolyte at optimum temperature and pH to form insoluble visible complex. This is named a precipitation reaction. It is often used for qualitative and quantitative determination of antigen and antibody. It includes the reaction of soluble antigen with soluble antibodies and form large interlocking aggravated which called lattice. It happens in two stages. Firstly, the antigen and antibody rapidly form antigen-antibody complexes in few seconds then it followed by a slower reaction in which the antibody-antigen complexes forms lattices which precipitate from the solution.
A ring test is useful for diagnosis of anthrax and detection of adulteration in food.
Agglutination reaction
It happens when antigen-antibody reaction cross-link particulate antigens and results in the visible clumping of the particle. There are two types active agglutination and passive agglutination. They are usually used in blood tests for diagnosis of enteric fever.
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