Antibody
An antibody (Ab) is the secreted form of a B cell receptor; the term immunoglobulin (Ig) can refer to either the membrane-bound form or the secreted form of the B cell receptor, but they are, broadly speaking, the same protein, and so the terms are often treated as synonymous.[1] Antibodies are large, Y-shaped proteins belonging to the immunoglobulin superfamily which are used by the immune system to identify and neutralize foreign objects such as bacteria and viruses, including those that cause disease. Antibodies can recognize virtually any size antigen with diverse chemical compositions from molecules.[2] Each antibody recognizes one or more specific antigens.[3][4] This term literally means "antibody generator", as it is the presence of an antigen that drives the formation of an antigen-specific antibody. Each tip of the "Y" of an antibody contains a paratope that specifically binds to one particular epitope on an antigen, allowing the two molecules to bind together with precision. Using this mechanism, antibodies can effectively "tag" a microbe or an infected cell for attack by other parts of the immune system, or can neutralize it directly (for example, by blocking a part of a virus that is essential for its invasion).
This article is about the class of proteins. For other uses, see Antibody (disambiguation).
To allow the immune system to recognize millions of different antigens, the antigen-binding sites at both tips of the antibody come in an equally wide variety. The rest of the antibody structure is relatively generic. In humans, antibodies occur in five classes, sometimes called isotypes: IgA, IgD, IgE, IgG, and IgM. Human IgG and IgA antibodies are also divided into discrete subclasses (IgG1, IgG2, IgG3, IgG4; IgA1 and IgA2). The class refers to the functions triggered by the antibody (also known as effector functions), in addition to some other structural features. Antibodies from different classes also differ in where they are released in the body and at what stage of an immune response. Importantly, while classes and subclasses of antibodies may be shared between species (at least in name), their functions and distribution throughout the body may be different. For example, mouse IgG1 is closer to human IgG2 than human IgG1 in terms of its function.
The term humoral immunity is often treated as synonymous with the antibody response, describing the function of the immune system that exists in the body's humors (fluids) in the form of soluble proteins, as distinct from cell-mediated immunity, which generally describes the responses of T cells (especially cytotoxic T cells). In general, antibodies are considered part of the adaptive immune system, though this classification can become complicated. For example, natural IgM,[5] which are made by B-1 lineage cells that have properties more similar to innate immune cells than adaptive, refers to IgM antibodies made independently of an immune response that demonstrate polyreactivity- they recognize multiple distinct (unrelated) antigens. These can work with the complement system in the earliest phases of an immune response to help facilitate clearance of the offending antigen and delivery of the resulting immune complexes to the lymph nodes or spleen for initiation of an immune response. Hence in this capacity, the function of antibodies is more akin to that of innate immunity than adaptive. Nonetheless, in general antibodies are regarded as part of the adaptive immune system because they demonstrate exceptional specificity (with some exception), are produced through genetic rearrangements (rather than being encoded directly in germline), and are a manifestation of immunological memory.
In the course of an immune response, B cells can progressively differentiate into antibody-secreting cells (B cells themselves do not secrete antibody; B cells do, however, express B cell receptors, the membrane-bound form of the antibody, on their surface) or memory B cells.[6] Antibody-secreting cells comprise plasmablasts and plasma cells, which differ mainly in the degree to which they secrete antibody, their lifespan, metabolic adaptations, and surface markers.[7] Plasmablasts are rapidly proliferating, short-lived cells produced in the early phases of the immune response (classically described as arising extrafollicularly rather than from the germinal center) which have the potential to differentiate further into plasma cells.[8] The literature is sloppy at times and often describes plasmablasts as just short-lived plasma cells- formally this is incorrect. Plasma cells, in contrast, do not divide (they are terminally differentiated), and rely on survival niches comprising specific cell types and cytokines to persist.[9] Plasma cells will secrete huge quantities of antibody regardless of whether or not their cognate antigen is present, ensuring that antibody levels to the antigen in question do not fall to 0, provided the plasma cell stays alive. The rate of antibody secretion, however, can be regulated, for example, by the presence of adjuvant molecules that stimulate the immune response such as TLR ligands.[10] Long-lived plasma cells can live for potentially the entire lifetime of the organism.[11] Classically, the survival niches that house long-lived plasma cells reside in the bone marrow,[12] though it cannot be assumed that any given plasma cell in the bone marrow will be long-lived. However, other work indicates that survival niches can readily be established within the mucosal tissues- though the classes of antibodies involved show a different hierarchy from those in the bone marrow.[13][14] B cells can also differentiate into memory B cells which can persist for decades similarly to long-lived plasma cells. These cells can be rapidly recalled in a secondary immune response, undergoing class switching, affinity maturation, and differentiating into antibody-secreting cells.
Antibodies are central to the immune protection elicited by most vaccines and infections (although other components of the immune system certainly participate and for some diseases are considerably more important than antibodies in generating an immune response, e.g. herpes zoster).[15] Durable protection from infections caused by a given microbe – that is, the ability of the microbe to enter the body and begin to replicate (not necessarily to cause disease) – depends on sustained production of large quantities of antibodies, meaning that effective vaccines ideally elicit persistent high levels of antibody, which relies on long-lived plasma cells. At the same time, many microbes of medical importance have the ability to mutate to escape antibodies elicited by prior infections, and long-lived plasma cells cannot undergo affinity maturation or class switching. This is compensated for through memory B cells: novel variants of a microbe that still retain structural features of previously encountered antigens can elicit memory B cell responses that adapt to those changes. It has been suggested that long-lived plasma cells secrete B cell receptors with higher affinity than those on the surfaces of memory B cells, but findings are not entirely consistent on this point.[16]
Antibody–antigen interactions[edit]
The antibody's paratope interacts with the antigen's epitope. An antigen usually contains different epitopes along its surface arranged discontinuously, and dominant epitopes on a given antigen are called determinants.
Antibody and antigen interact by spatial complementarity (lock and key). The molecular forces involved in the Fab-epitope interaction are weak and non-specific – for example electrostatic forces, hydrogen bonds, hydrophobic interactions, and van der Waals forces. This means binding between antibody and antigen is reversible, and the antibody's affinity towards an antigen is relative rather than absolute. Relatively weak binding also means it is possible for an antibody to cross-react with different antigens of different relative affinities.
Medical applications[edit]
Disease diagnosis[edit]
Detection of particular antibodies is a very common form of medical diagnostics, and applications such as serology depend on these methods.[98] For example, in biochemical assays for disease diagnosis,[99] a titer of antibodies directed against Epstein-Barr virus or Lyme disease is estimated from the blood. If those antibodies are not present, either the person is not infected or the infection occurred a very long time ago, and the B cells generating these specific antibodies have naturally decayed.
In clinical immunology, levels of individual classes of immunoglobulins are measured by nephelometry (or turbidimetry) to characterize the antibody profile of patient.[100] Elevations in different classes of immunoglobulins are sometimes useful in determining the cause of liver damage in patients for whom the diagnosis is unclear.[1] For example, elevated IgA indicates alcoholic cirrhosis, elevated IgM indicates viral hepatitis and primary biliary cirrhosis, while IgG is elevated in viral hepatitis, autoimmune hepatitis and cirrhosis.
Autoimmune disorders can often be traced to antibodies that bind the body's own epitopes; many can be detected through blood tests. Antibodies directed against red blood cell surface antigens in immune mediated hemolytic anemia are detected with the Coombs test.[101] The Coombs test is also used for antibody screening in blood transfusion preparation and also for antibody screening in antenatal women.[101]
Practically, several immunodiagnostic methods based on detection of complex antigen-antibody are used to diagnose infectious diseases, for example ELISA, immunofluorescence, Western blot, immunodiffusion, immunoelectrophoresis, and magnetic immunoassay.[102] Antibodies raised against human chorionic gonadotropin are used in over the counter pregnancy tests.
New dioxaborolane chemistry enables radioactive fluoride (18F) labeling of antibodies, which allows for positron emission tomography (PET) imaging of cancer.[103]
Disease therapy[edit]
Targeted monoclonal antibody therapy is employed to treat diseases such as rheumatoid arthritis,[104] multiple sclerosis,[105] psoriasis,[106] and many forms of cancer including non-Hodgkin's lymphoma,[107] colorectal cancer, head and neck cancer and breast cancer.[108]
Some immune deficiencies, such as X-linked agammaglobulinemia and hypogammaglobulinemia, result in partial or complete lack of antibodies.[109] These diseases are often treated by inducing a short-term form of immunity called passive immunity. Passive immunity is achieved through the transfer of ready-made antibodies in the form of human or animal serum, pooled immunoglobulin or monoclonal antibodies, into the affected individual.[110]
Prenatal therapy[edit]
Rh factor, also known as Rh D antigen, is an antigen found on red blood cells; individuals that are Rh-positive (Rh+) have this antigen on their red blood cells and individuals that are Rh-negative (Rh–) do not. During normal childbirth, delivery trauma or complications during pregnancy, blood from a fetus can enter the mother's system. In the case of an Rh-incompatible mother and child, consequential blood mixing may sensitize an Rh- mother to the Rh antigen on the blood cells of the Rh+ child, putting the remainder of the pregnancy, and any subsequent pregnancies, at risk for hemolytic disease of the newborn.[111]
Rho(D) immune globulin antibodies are specific for human RhD antigen.[112] Anti-RhD antibodies are administered as part of a prenatal treatment regimen to prevent sensitization that may occur when a Rh-negative mother has a Rh-positive fetus. Treatment of a mother with Anti-RhD antibodies prior to and immediately after trauma and delivery destroys Rh antigen in the mother's system from the fetus. This occurs before the antigen can stimulate maternal B cells to "remember" Rh antigen by generating memory B cells. Therefore, her humoral immune system will not make anti-Rh antibodies, and will not attack the Rh antigens of the current or subsequent babies. Rho(D) Immune Globulin treatment prevents sensitization that can lead to Rh disease, but does not prevent or treat the underlying disease itself.[112]
Regulations[edit]
Production and testing[edit]
There are several ways to obtain antibodies, including in vivo techniques like animal immunization and various in vitro approaches, such as the phage display method.[129] Traditionally, most antibodies are produced by hybridoma cell lines through immortalization of antibody-producing cells by chemically induced fusion with myeloma cells. In some cases, additional fusions with other lines have created "triomas" and "quadromas". The manufacturing process should be appropriately described and validated. Validation studies should at least include:
Structure prediction and computational antibody design[edit]
The importance of antibodies in health care and the biotechnology industry demands knowledge of their structures at high resolution. This information is used for protein engineering, modifying the antigen binding affinity, and identifying an epitope, of a given antibody. X-ray crystallography is one commonly used method for determining antibody structures. However, crystallizing an antibody is often laborious and time-consuming. Computational approaches provide a cheaper and faster alternative to crystallography, but their results are more equivocal, since they do not produce empirical structures. Online web servers such as Web Antibody Modeling (WAM)[130] and Prediction of Immunoglobulin Structure (PIGS)[131] enable computational modeling of antibody variable regions. Rosetta Antibody is a novel antibody FV region structure prediction server, which incorporates sophisticated techniques to minimize CDR loops and optimize the relative orientation of the light and heavy chains, as well as homology models that predict successful docking of antibodies with their unique antigen.[132] However, describing an antibody's binding site using only one single static structure limits the understanding and characterization of the antibody's function and properties. To improve antibody structure prediction and to take the strongly correlated CDR loop and interface movements into account, antibody paratopes should be described as interconverting states in solution with varying probabilities.[27]
The ability to describe the antibody through binding affinity to the antigen is supplemented by information on antibody structure and amino acid sequences for the purpose of patent claims.[133] Several methods have been presented for computational design of antibodies based on the structural bioinformatics studies of antibody CDRs.[134][135][136]
There are a variety of methods used to sequence an antibody including Edman degradation, cDNA, etc.; albeit one of the most common modern uses for peptide/protein identification is liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS).[137] High volume antibody sequencing methods require computational approaches for the data analysis, including de novo sequencing directly from tandem mass spectra[138] and database search methods that use existing protein sequence databases.[139][140] Many versions of shotgun protein sequencing are able to increase the coverage by utilizing CID/HCD/ETD[141] fragmentation methods and other techniques, and they have achieved substantial progress in attempt to fully sequence proteins, especially antibodies. Other methods have assumed the existence of similar proteins,[142] a known genome sequence,[143] or combined top-down and bottom up approaches.[144] Current technologies have the ability to assemble protein sequences with high accuracy by integrating de novo sequencing peptides, intensity, and positional confidence scores from database and homology searches.[145]
Antibody mimetic[edit]
Antibody mimetics are organic compounds, like antibodies, that can specifically bind antigens. They consist of artificial peptides or proteins, or aptamer-based nucleic acid molecules with a molar mass of about 3 to 20 kDa. Antibody fragments, such as Fab and nanobodies are not considered as antibody mimetics. Common advantages over antibodies are better solubility, tissue penetration, stability towards heat and enzymes, and comparatively low production costs. Antibody mimetics have being developed and commercialized as research, diagnostic and therapeutic agents.[146]