process of generating antibodies with increased binding affinities is called affinity maturation. affinity maturation occurs in mature b cells after v(d)j recombination, and is dependent on help from helper t cells. class switching mechanism of class switch recombination that allows isotype switching in activated b cells isotype or class switching is a biological process occurring after activation of the b cell, which allows the cell to produce different classes of antibody (iga, ige, or igg). the different classes of antibody, and thus effector functions, are defined by the constant (c) regions of the immunoglobulin heavy chain. initially, naive b cells express only cell-surface igm and igd with identical antigen binding regions. each isotype is adapted for a distinct function; therefore, after activation, an antibody with an igg, iga, or ige effector function might be required to effectively eliminate an antigen. class switching allows different daughter cells from the same activated b cell to produce antibodies of different isotypes. only the constant region of the antibody heavy chain changes during class switching; the variable regions, and therefore antigen specificity, remain unchanged. thus the progeny of a single b cell can produce antibodies, all specific for the same antigen, but with the ability to produce the effector function appropriate for each antigenic challenge. class switching is triggered by cytokines; the isotype generated depends on which cytokines are present in the b cell environment. class switching occurs in the heavy chain gene locus by a mechanism called class switch recombination (csr). this mechanism relies on conserved nucleotide motifs, called switch (s) regions, found in dna upstream of each constant region gene (except in the δ-chain). the dna strand is broken by the activity of a series of enzymes at two selected s-regions. the variable domain exon is rejoined through a process called non-homologous end joining (nhej) to the desired constant region (γ, α or ε). this process results in an immunoglobulin gene that encodes an antibody of a different isotype. specificity designations an antibody can be called monospecific if it has specificity for the same antigen or epitope, or bispecific if they have affinity for two different antigens or two different epitopes on the same antigen. a group of antibodies can be called polyvalent (or unspecific) if they have affinity for various antigens or microorganisms. intravenous immunoglobulin, if not otherwise noted, consists of a variety of different igg (polyclonal igg). in contrast, monoclonal antibodies are identical antibodies produced by a single b cell. asymmetrical antibodies heterodimeric antibodies, which are also asymmetrical antibodies, allow for greater flexibility and new formats for attaching a variety of drugs to the antibody arms. one of the general formats for a heterodimeric antibody is the "knobs-into-holes" format. this format is specific to the heavy chain part of the constant region in antibodies. the "knobs" part is engineered by replacing a small amino acid with a larger one. it fits into the "hole", which is engineered by replacing a large amino acid with a smaller one. what connects the "knobs" to the "holes" are the disulfide bonds between each chain. the "knobs-into-holes" shape facilitates antibody dependent cell mediated cytotoxicity. single chain variable fragments (scfv) are connected to the variable domain of the heavy and light chain via a short linker peptide. the linker is rich in glycine, which gives it more flexibility, and serine/threonine, which gives it specificity. two different scfv fragments can be connected together, via a hinge region, to the constant domain of the heavy chain or the constant domain of the light chain. this gives the antibody bispecificity, allowing for the binding specificities of two different antigens. the "knobs-into-holes" format enhances heterodimer formation but doesn't suppress homodimer formation. to further improve the function of heterodimeric antibodies, many scientists are looking towards artificial constructs. artificial antibodies are largely diverse protein motifs that use the functional strategy of the antibody molecule, but aren't limited by the loop and framework structural constraints of the natural antibody. being able to control the combinational design of the sequence and three-dimensional space could transcend the natural design and allow for the attachment of different combinations of drugs to the arms. heterodimeric antibodies have a greater range in shapes they can take and the drugs that are attached to the arms don't have to be the same on each arm, allowing for different combinations of drugs to be used in cancer treatment. pharmaceuticals are able to produce highly functional bispecific, and even multispecific, antibodies. the degree to which they can function is impressive given that such a change of shape from the natural form should lead to decreased functionality. history the first use of the term "antibody" occurred in a text by paul ehrlich. the term antikörper (the german word for antibody) appears in the conclusion of his article "experimental studies on immunity", published in october 1891, which states that, "if two substances give rise to two different antikörper, then they themselves must be different". however, the term was not accepted immediately and several other terms for antibody were proposed; these included immunkörper, amboceptor, zwischenkörper, substance sensibilisatrice, copula, desmon, philocytase, fixateur, and immunisin. the word antibody has formal analogy to the word antitoxin and a similar concept to immunkörper (immune body in english). as such, the original construction of the word contains a logical flaw; the antitoxin is something directed against a toxin, while the antibody is a body directed against something. angel of the west (2008) by julian voss-andreae is a sculpture based on the antibody structure published by e. padlan. created for the florida campus of the scripps research institute, the antibody is placed into a ring referencing leonardo da vinci's vitruvian man thus highlighting the similarity of the antibody and the human body. the study of antibodies began in 1890 when emil von behring and kitasato shibasaburō described antibody activity against diphtheria and tetanus toxins. von behring and kitasato put forward the theory of humoral immunity, proposing that a mediator in serum could react with a foreign antigen. his idea prompted paul ehrlich to propose the side-chain theory for antibody and antigen interaction in 1897, when he hypothesized that receptors (described as "side-chains") on the surface of cells could bind specifically to toxins – in a "lock-and-key" interaction – and that this binding reaction is the trigger for the production of antibodies. other researchers believed that antibodies existed freely in the blood and, in 1904, almroth wright suggested that soluble antibodies coated bacteria to label them for phagocytosis and killing; a process that he named opsoninization. michael heidelberger in the 1920s, michael heidelberger and oswald avery observed that antigens could be precipitated by antibodies and went on to show that antibodies are made of protein. the biochemical properties of antigen-antibody-binding interactions were examined in more detail in the late 1930s by john marrack. the next major advance was in the 1940s, when linus pauling confirmed the lock-and-key theory proposed by ehrlich by showing that the interactions between antibodies and antigens depend more on their shape than their chemical composition. in 1948, astrid fagraeus discovered that b cells, in the form of plasma cells, were responsible for generating antibodies. further work concentrated on characterizing the structures of the antibody proteins. a major advance in these structural studies was the discovery in the early 1960s by gerald edelman and joseph gally of the antibody light chain, and their realization that this protein is the same as the bence-jones protein described in 1845 by henry bence jones. edelman went on to discover that antibodies are composed of disulfide bond-linked heavy and light chains. around the same time, antibody-binding (fab) and antibody tail (fc) regions of igg were characterized by rodney porter. together, these scientists deduced the structure and complete amino acid sequence of igg, a feat for which they were jointly awarded the 1972 nobel prize in physiology or medicine. the fv fragment was prepared and characterized by david givol. while most of these early studies focused on igm and igg, other immunoglobulin isotypes were identified in the 1960s: thomas tomasi discovered secretory antibody (iga); david s. rowe and john l. fahey discovered igd; and kimishige ishizaka and teruko ishizaka discovered ige and showed it was a class of antibodies involved in allergic reactions. in a landmark series of experiments beginning in 1976, susumu tonegawa showed that genetic material can rearrange itself to form the vast array of available antibodies. medical applications disease diagnosis detection of particular antibodies is a very common form of medical diagnostics, and applications such as serology depend on these methods. for example, in biochemical assays for disease diagnosis, 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. elevations in different classes of immunoglobulins are sometimes useful in determining the cause of liver damage in patients for whom the diagnosis is unclear. 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. the coombs test is also used for antibody screening in blood transfusion preparation and also for antibody screening in antenatal women. 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. 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. disease therapy targeted monoclonal antibody therapy is employed to treat diseases such as rheumatoid arthritis, multiple sclerosis, psoriasis, and many forms of cancer including non-hodgkin's lymphoma, colorectal cancer, head and neck cancer and breast cancer. some immune deficiencies, such as x-linked agammaglobulinemia and hypogammaglobulinemia, result in partial or complete lack of antibodies. 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. prenatal therapy 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. rho(d) immune globulin antibodies are specific for human rhd antigen. 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. it is important to note that 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. research applications immunofluorescence image of the eukaryotic cytoskeleton. microtubules as shown in green, are marked by an antibody conjugated to a green fluorescing molecule, fitc. specific antibodies are produced by injecting an antigen into a mammal, such as a mouse, rat, rabbit, goat, sheep, or horse for