The main categories of<unk>  action include the following: * Neutralisation, in which neutralizing<unk> es block parts of the surface of a bacterial cell or virion to render its attack ineffective * Agglutination, in which<unk> es "glue together" foreign cells into clumps that are attractive targets for phagocytosis * Precipitation, in which<unk> es "glue together" serum-soluble antigens, forcing them to precipitate out of solution in clumps that are attractive targets for phagocytosis * Complement activation (fixation), in which<unk> es that are latched onto a foreign cell encourage complement to attack it with a membrane attack complex, which leads to the following: * Lysis of the foreign cell * Encouragement of inflammation by chemotactically attracting inflammatory cells More indirectly, an<unk>  can signal immune cells to present<unk>  fragments to T cells, or downregulate other immune cells to avoid autoimmunity. Activated B cells differentiate into either<unk> -producing cells called plasma cells that secrete soluble<unk>  or memory cells that survive in the body for years afterward in order to allow the immune system to remember an antigen and respond faster upon future exposures. At the prenatal and neonatal stages of life, the presence of<unk> es is provided by passive immunization from the mother.<unk> rly endogenous<unk>  production varies for different kinds of<unk> es, and usually appear within the first years of life. Since<unk> es exist freely in the bloodstream, they are said to be part of the humoral immune system. Circulating<unk> es are produced by clonal B cells that specifically respond to only one antigen (an example is a virus capsid protein fragment).<unk> es contribute to immunity in three ways: They prevent pathogens from entering or damaging cells by binding to them; they stimulate removal of pathogens by macrophages and other cells by coating the pathogen; and they trigger destruction of pathogens by stimulating other immune responses such as the complement pathway.<unk> es will also trigger vasoactive amine degranulation to contribute to immunity against certain types of antigens (helminths, allergens). The secreted mammalian IgM<unk> s five Ig units.<unk> ch Ig unit (labeled 1)<unk> s two epitope binding<unk>  regions, so IgM is capable of binding up to 10 epitopes. Activation of complement<unk> es that bind to surface antigens (for example, on bacteria) will attract the first component of the complement cascade with their Fc region and initiate activation of the "classical" complement system. This results in the killing of bacteria in two ways. First, the binding of the<unk>  and complement molecules marks the microbe for ingestion by phagocytes in a process called opsonization; these phagocytes are attracted by certain complement molecules generated in the complement cascade. Second, some complement system components form a membrane attack complex to assist<unk> es to kill the bacterium directly (bacteriolysis). Activation of effector cells To combat pathogens that replicate outside cells,<unk> es bind to pathogens to link them together, causing them to agglutinate. Since an<unk> <unk> s at least two paratopes, it can bind more than one antigen by binding identical epitopes carried on the surfaces of these antigens. By coating the pathogen,<unk> es stimulate effector functions against the pathogen in cells that recognize their Fc region. Those cells that recognize coated pathogens<unk> ve Fc receptors, which, as the name suggests, interact with the Fc region of IgA, IgG, and<unk> <unk> es. The engagement of a particular<unk>  with the Fc receptor on a particular cell triggers an effector function of that cell; phagocytes will phagocytose, mast cells and neutrophils will degranulate, natural killer cells will release cytokines and cytotoxic molecules; that will ultimately result in destruction of the invading microbe. The activation of natural killer cells by<unk> es initiates a cytotoxic mechanism known as<unk> -dependent cell-mediated cytotoxicity (ADCC) – this process may explain the efficacy of monoclonal<unk> es used in biological therapies against cancer. The Fc receptors are isotype-specific, which gives greater flexibility to the immune system, invoking only the appropriate immune mechanisms for distinct pathogens. Natural<unk> es Humans and higher primates also produce "natural<unk> es" that are present in serum before viral infection. Natural<unk> es<unk> ve been defined as<unk> es that are produced without any previous infection, vaccination, other foreign antigen exposure or passive immunization. These<unk> es can activate the classical complement pathway leading to lysis of enveloped virus particles long before the adaptive immune response is activated. Many natural<unk> es are directed against the disaccharide galactose α(1,3)-galactose (α-Gal), which is found as a terminal sugar on glycosylated cell surface proteins, and generated in response to production of this sugar by bacteria contained in the human gut. Rejection of xenotransplantated organs is thought to be, in part, the result of natural<unk> es circulating in the serum of the recipient binding to α-Gal antigens expressed on the donor tissue.
Antibody
Microantibody
Heterophile antibody
Antigen
Isoantibodies
Immune complex
Autoantibody
Immunogen