INTRODUCTION — The basic aspects of immunoglobulin structure will be reviewed here. The terms "immunoglobulin" and "antibody" are generally used interchangeably, although "immunoglobulin" is preferred in this review. Discussions of immunoglobulin genetics, the humoral immune response, and the derivation of therapeutic monoclonal antibodies are presented separately:
●(See "Immunoglobulin genetics".)
HISTORICAL BACKGROUND — The 1972 Nobel Prize in Physiology or Medicine was awarded jointly to Gerald M Edelman and Rodney R Porter "for their discoveries concerning the chemical structure of antibodies" [1,2]. Porter's work used the protein-splitting enzyme papain to identify three fragments, two smaller very similar ones, both with capacity of combining with the antigen, and one larger piece lacking this capacity. Edelman's contribution was the demonstration that immunoglobulin molecules, like most biologically active proteins, were composed of chain structures that were held together by sulfur bonds and could be separated. None of the resultant fragments retained the specific reactivity of the intact immunoglobulin molecule. Since then, progress has been made in understanding the finer characteristics of the various domains of the individual chains of the immunoglobulin molecule.
Components — The components that make up the immunoglobulin structure are the following:
●Two identical heavy (H) chains that are glycosylated (ie, have associated carbohydrate moieties) – Each chain has a variable (V) region at the amino (NH2) end and a constant (C) region at the carboxyl end. The molecular weight (MW) of each heavy chain is 55 kilodaltons (Kd), and it contains approximately 440 or 550 amino acids, depending on the isotype. There are five distinct heavy chains: gamma, alpha, mu, epsilon, and delta.
●Two identical light (L) chains that do not have carbohydrate moieties – Similar to the heavy chains, light chains have a variable and a constant region as well. The MW of each light chain is 23 Kd, and it contains approximately 220 amino acids. There are only two distinct light chains in humans: kappa or lambda. Each individual immunoglobulin molecule will have either two kappa or two lambda chains. The frequency of the kappa chain is much higher than the lambda chain, with a ratio of 2:1.
•Intrachain – These bonds are within either the light or heavy chain and allow the folding of the immunoglobulin molecule into its "globular" structure (figure 1).
•Interchain – The disulfide bonds that bind the heavy chains together are located in the hinge region of the antibody molecules. These bonds are targeted by enzymatic digestion for generation of various antibody fragments, as discussed below. There is also at least one disulfide bond between each heavy and light chain .
●Hinge region – The hinge region is a proline-rich sequence that confers flexibility on the immunoglobulin molecule to ensure better antigen accommodation (figure 1). A hinge region is found in IgG, IgA, and IgD but not in IgM or IgE. The length of the hinge region differs in various IgG subclasses (IgG1 through IgG4). Proteases (such as papain or pepsin) cleave immunoglobulin molecules at this region, resulting in the fragments discussed below . (See 'Fab fragment' below and 'Fc fragment' below and 'F(ab')2' below.)
●Carbohydrate moieties – Immunoglobulin molecules are glycoproteins. The carbohydrate moieties are usually associated with the second constant region of the heavy chain (CH2) domain of the heavy chain but can be at other sites (figure 1). Glycosylation plays an important role in the molecule's secretion, solubility, and interaction with receptors and complement proteins .
●The joining chain – The joining chain is a small polypeptide with an approximate MW of 15 Kd, which has cysteine residues that link the heavy chains of IgM (to form a pentamer or hexamer) or the heavy chains of IgA (to form a dimer) by disulfide bridges [9,10].
●Polymerized immunoglobulin receptor (pIgR) or secretory component (SC) – pIgR or SC is produced by epithelial cells. Polymerized IgA (formed by the adjoining of two IgA molecules by the joining chain) binds to pIgR found in the basolateral epithelial surfaces, allowing transport of the IgA dimer to the apical surface of the epithelial cells (ie, transcytosis) and secretion onto the mucosal surface. Release into the mucosal lumen requires splitting of the pIgR from the IgA molecule [11,12].
Three-dimensional structure — Although immunoglobulin molecules are depicted in two-dimensional drawings, it is important to remember that they fold into a tertiary structure, which results into globular regions called domains (figure 2). There is an intrachain disulfide bond in each domain. Each light chain has two domains (VL and CL), and each heavy chain has four or five domains, depending on the isotype (VH and CH1-CH3 or CH1-CH4) [13-15].
Secreted versus membrane bound — Immunoglobulins exist in a secreted form and a membrane-bound form on the surface of the B cells. The membrane-bound but not the secreted form has a hydrophobic transmembrane segment that anchors the molecule to the B cell membrane. This is discussed in more detail elsewhere. (See "Immunoglobulin genetics", section on 'Forms of immunoglobulin'.)
MEASUREMENT OF IMMUNOGLOBULINS — Electrophoresis of plasma proteins yields albumin, alpha-1, alpha-2, beta, and gamma regions. Most of the immunoglobulin molecules are in the gamma region (figure 3) . The amplitude and shape of the gamma region can indicate hypogammaglobulinemia (a broad peak with decreased amplitude) or a monoclonal gammopathy (a narrow peak with increased amplitude that looks similar to the albumin peak) (figure 4).
Nephelometry is the gold standard for measuring IgG, IgA, IgM, and IgD. IgE is measured by immunoassay. Normal ranges vary by patient age and among different laboratories, and clinicians should rely upon the reference ranges provided by the performing facility. (See "Laboratory evaluation of the immune system", section on 'Measurement of antibody levels'.)
Hypervariable region/complementarity-determining region — As mentioned above, both heavy and light chains have a variable region and a constant region. The variable region determines antigen specificity, while the constant region is important in binding to Fc receptors and complement components.
The variable region is composed of hypervariable and framework areas. At the amino terminal of each variable region is a hypervariable region, also called complementarity-determining region (CDR). There are CDRs for both the light and heavy chains of a given immunoglobulin molecule. Each chain has three CDRs. The various CDRs of each molecule are interspersed with framework regions, which account for >80 percent of the variable area. The CDRs of the heavy and light chain configure into a cleft that serves as the antigen-binding area. CDRs, therefore, determine antigen specificity. The dimerized hypervariable CDRs of a single heavy and a single light chain are referred to as the Fv fragment [16,17]. (See 'Idiotopes and idiotypes' below.)
Fragments of enzymatic digestion — The intact immunoglobulin molecule can be divided into different fragments, the structure of which is determined by where an enzyme splits the molecule (figure 5).
Fab fragment — Papain splits the molecule proximal (looking from the amino terminal end of the molecule) to the interchain disulfide bonds in the hinge region, resulting in two separate Fab fragments, each of which has a light chain connected with a sulfide bond to a fragment of the heavy chain, which contains the VH region and the CH1 region. Each Fab fragment can bind antigen but cannot bind the Fc receptor . Examples of medical therapies based on Fab fragments include multiple antivenom preparations and therapies for digitalis toxicity (ie, digitoxin immune Fab) .
Fc fragment — The part of the immunoglobulin molecule that remains after digestion with papain is called the Fc fragment and consists of the remainder of the heavy chain. This is a dimer since the two chains are connected by interchain disulfide bonds. The Fc fragment binds to the Fc receptors of various effector cells. In clinical medicine, Fc fragments can be fused to various proteins with therapeutic functions . For example, etanercept is produced by fusing the Fc portion of IgG1 to two tumor necrosis factor-alpha receptors. (See "Overview of therapeutic monoclonal antibodies" and "Overview of therapeutic monoclonal antibodies", section on 'IgG1 fusion proteins'.)
F(ab')2 — Digestion with pepsin splits the immunoglobulin molecule distal to the interchain disulfide bonds, resulting in a fragment that contains two Fab fragments connected by interchain sulfide bonds. The remaining part of the heavy chains are frequently referred to as Fc fragments, but this is not quite true, since they are monovalent fragments of the remainder of the heavy chains that do not bind antigen and do not bind Fc receptors (figure 5). F(ab')2 fragments are also used as antivenoms (eg, ovine polyvalent Crotalidae North and South American snake antivenom).
IMMUNOGLOBULIN ISOTYPES — An immunoglobulin isotype is determined by antigenic determinants in the constant region of the heavy chains. In contrast, the variable region is encoded by gene segments that are distinct from those that code the constant regions and are not considered in identifying an immunoglobulin isotype.
There are five isotypes: IgG (the heavy chain is designated gamma), IgA (alpha), IgM (mu), IgE (epsilon), and IgD (delta). Immunoglobulins are glycoproteins and thus can serve as antigens. This property was used to identify the various immunoglobulin isotypes.
The immunoglobulin isotypes can further be designated as having either a kappa or a lambda light chain. This nomenclature is important in identifying and describing monoclonal antibodies and myeloma proteins. For example, the full designation of an immunoglobulin would be IgG-kappa or IgA-lambda. Although the basic structure is identical for the various isotypes, there are characteristics that may be unique to some but not all isotypes (table 1) .
IgG — IgG is a monomer (7S immunoglobulin) and is the most abundant isotype in the body. A normal serum level is between 700 and 1600 mg/dL (exact range may vary in different laboratories). Although IgG has the highest concentration in the serum, it is generally thought of as being relatively more abundant in the tissues. Its mean half-life is 21 days. IgG has a molecular weight (MW) of 150 kilodaltons (Kd).
IgG is the only isotype that crosses the placenta, and this is accomplished by binding to the neonatal Fc receptor (table 1). IgG has four subclasses whose numeric designation indicates their relative serum concentration. The four subclasses are mainly differentiated by the number of interchain disulfide bonds in their hinge region. This allows distinct flexibility properties. The subclasses also show functional disparities in their ability to fix complement, their ability to bind the Fc receptors, and their ability to cross the placenta. (See "IgG subclasses: Physical properties, genetics, and biologic functions".)
IgA — IgA accounts for 10 to 15 percent of serum immunoglobulins. Its normal serum concentration ranges from 70 to 400 mg/dL. It is the dominant antibody at the mucosal surfaces and is thus abundant in saliva, tears, nasal secretion, bronchial secretions, and in the gastrointestinal tract. IgA has the highest rate of synthesis of all the isotypes but has a half-life of six days. Serum IgA is monomeric and has a MW of 160 Kd. Secretory IgA is dimeric, with the joining chain connecting the two IgA molecules. Secretory IgA uses the polymerized immunoglobulin receptor or secretory component for secretion. IgA has two subclasses: IgA1 and IgA2. Eighty percent of serum IgA is IgA1, while the majority of secretory IgA is IgA2. Monomeric IgA does not fix complement, but dimeric IgA can do so, usually via the alternative pathway. IgA1 may be more efficient at complement activation. IgA can bind to some cells (some lymphocytes and neutrophils) through its Fc receptor. IgA is discussed in more detail elsewhere. (See "Structure and biologic functions of IgA".)
IgM — IgM accounts for 5 to 10 percent of serum immunoglobulins. It is mostly intravascular. It is a monomer on the surface of B cells but is a pentamer when secreted. IgM hexamers have been reported, but their biologic significance is unknown. Pentameric IgM is formed by covalent linkage of individual molecules with the joining chain. The normal serum concentration of IgM is 50 to 400 mg/dL. As a pentamer, it has a MW of 950 Kd and fixes complement. It binds Fc receptors. Its half-life is five days. IgM has an extra heavy chain domain (CH4).
IgM that is on the surface of B cells is noncovalently associated with two additional chains on the B cell surface (Ig-alpha and Ig-beta). Membrane-bound IgM cannot induce signal transduction, although it can do so through association with these two chains.
IgE — IgE has a very low serum concentration and accounts for <1 percent of serum immunoglobulins. Its concentration can be expressed as weight or as international units with one international unit being equivalent to 2.4 ng. This may cause some confusion in the literature, as any given paper usually uses one or the other scale but not both. The normal serum concentration is 0 to 100 international units/mL or 0 to 250 mcg/L (sometimes expressed in ng/mL). Like IgM, it has an extra heavy chain domain (CH4). It is a heavily glycosylated monomer and is found mostly in the tissue bound to the surface of mast cells. Its serum half-life is two days, but its tissue half-life is probably much longer, although no real estimates are available for the latter. It does not fix complement or cross the placenta. (See "The biology of IgE".)
IgD — IgD is a monomer but is barely detectable in normal human serum. IgD is mainly expressed on the surface of mature, naïve B cells, where it has extra amino acids at the C-terminal, allowing it to anchor into the cell membrane. Like IgM on the B cell surface, it associates with the Ig-alpha and Ig-beta chains. It has a MW of 170 Kd and a half-life of three days. Very little is known about its function, but elevated levels (hyper-IgD syndrome) are associated with an autoinflammatory clinical picture. There are some data implicating IgD in activating basophils and mast cells. (See "Hyperimmunoglobulin D syndrome: Clinical manifestations and diagnosis".)
ALLOTYPES — Some of the Mendelian genes encoding the constant region of heavy and light chains differ between individuals and are said to be allotypic (derived from the Greek word "allos" meaning different or "other"). Allotypic determinants can be identified by immunizing one individual with immunoglobulin molecules from another individual in the same species.
Allotypes have been described for IgG (all four subclasses) and IgA2 heavy chains as well as for the kappa-light chain. The nomenclature consists of a capital letter that indicates the isotype, followed by the number of the subclass, then the letter "m" (short for "marker"), followed by a number in parenthesis, which indicates the particular allele. Therefore a G2m(4) designation indicates allotype 4 for IgG2 subclass. The amino acids determining the allotype are found in the distal part of the constant regions of the heavy and light chains (figure 6) . Allotyping has been used in paternity testing .
IDIOTOPES AND IDIOTYPES — The determinant on an antigen that is recognized by an antibody is called an epitope. The hypervariable part of the antibody that binds the epitope is called a paratope. Given the almost infinite possibilities of antibody specificities, an antibody against a certain antigen will have minor changes in the amino acids in the hypervariable region (the Fv region). This "new" configuration can be seen as an antigen by the immune system and is called an idiotope.
Depending on the configuration of the various amino acid changes in the variable region, a given antibody can have more than one idiotope. The collective expression of these idiotopes determines the idiotype of an antibody. Anti-idiotype preparations of immune globulin raised against anti-DNA or antiphospholipid antibodies as well as synthetic idiotypes have been studied for the treatment of systemic lupus erythematosus .
One can raise anti-idiotype antibodies and anti-anti-idiotype antibodies in an almost never-ending sequence. The 1984 Nobel Prize in Physiology or Medicine was shared by Niels K Jerne, Georges JF Köhler, and César Milstein. Jerne was recognized for his theory that "in 1975, described how the immune response is exquisitely controlled and was built on his premise that antibodies can themselves act as antigens" .
●Basic immunoglobulin structure – Each immunoglobulin molecule has two heavy chains and two light chains (figure 1). Each chain has a variable region and a constant region. At the amino terminal of the variable region, there is a hypervariable area or complementarity-determining region where antigen is recognized and bound. (See 'Structure' above.)
●Fab and Fc fragments – The Fab portions of the molecule bind antigen, while the Fc portion binds to Fc receptors on various effector cells (figure 2). Papain digestion of an immunoglobulin molecule results in two Fab fragments and an Fc fragment (figure 5). Pepsin digestion results in one F(ab')2 segment and fragmented remnants of the heavy chain. (See 'Fragments of enzymatic digestion' above.)
●Five isotypes – There are five immunoglobulin isotypes: IgG, IgA, IgM, IgE, and IgD (table 1). The immunoglobulin isotypes can further be designated as having either a kappa or a lambda light chain, such that the full designation of an immunoglobulin would be IgG-kappa or IgA-lambda. IgG, IgE, and IgD are monomeric. IgA can be monomeric or dimeric. IgM is a pentamer but can be a hexamer. The polymerization of IgA and IgM is made possible by the joining chain, which joins individual immunoglobulin molecules together. (See 'Immunoglobulin isotypes' above.)
●Allotypes – Differences in the genes encoding the constant regions of heavy and light chains give rise to differences between similar antibody molecules from two different individuals of the same species, which are called allotypes (figure 6). (See 'Allotypes' above.)
●Idiotypes – Differences in the amino acid sequence of the hypervariable regions of all of the antibodies an individual has against a given antigen are called idiotopes. The collective expression of all the idiotopes against a given antigen is called an idiotype. (See 'Idiotopes and idiotypes' above.)
ACKNOWLEDGMENTS — The UpToDate editorial staff acknowledges Francisco A Bonilla, MD, PhD, who contributed to an earlier version of this topic review.
The UpToDate editorial staff also acknowledges E Richard Stiehm, MD, who contributed as a Section Editor to an earlier version of this topic review.
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