Antibodies

Antibodies.

An?antibody?(Ab), also known as an?immunoglobulin?(Ig),?is a large, Y-shaped?protein?produced mainly by?plasma cells?that is used by the?immune system?to identify and neutralize pathogens such as?bacteria?and?viruses. The antibody recognizes a unique molecule of the harmful agent, called an?antigen, via the variable region.

Each tip of the "Y" of an antibody contains a?paratope (analogous to a lock) that is specific for one particular?epitope?(similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can?tag?a?microbe?or an infected cell for attack by other parts of the immune system, or can neutralize its target directly (for example, by blocking a part of a microbe that is essential for its invasion and survival).

Depending on the antigen, the binding may impede the biological process causing the disease or may recruit macrophages to destroy the foreign substance. The ability of an antibody to communicate with the other components of the immune system is mediated via its Fc region (located at the base of the "Y"), which contains a conserved?glycosylation?site involved in these interactions.?The production of antibodies is the main function of the?humoral immune system.

Secretion of antibodies.

Antibodies are secreted by?B cells?of the adaptive immune system, mostly by differentiated B cells called?plasma cells. Antibodies can occur in two physical forms, a soluble form that is secreted from the cell to be free in the?blood plasma, and a?membrane-bound form that is attached to the surface of a?B cell?and is referred to as the?B-cell receptor?(BCR). The BCR is found only on the surface of B cells and facilitates the activation of these cells and their subsequent differentiation into either antibody factories called?plasma cells?or?memory B cells?that will survive in the body and remember that same antigen so the B cells can respond faster upon future exposure.?

In most cases, interaction of the B cell with a?T helper cell?is necessary to produce full activation of the B cell and, therefore, antibody generation following antigen binding.?Soluble antibodies are released into the?blood?and tissue fluids, as well as many?secretions?to continue to survey for invading microorganisms.

Composition of antibodies.

Antibodies are?glycoproteins?belonging to the?immunoglobulin superfamily.?They constitute most of the?gamma globulin?fraction of the?blood proteins. They are typically made of basic structural units—each with two large?heavy chains?and two small?light chains. There are several different types of antibody heavy chains based on five different types of crystallisable fragments (Fc) that may be attached to the antigen-binding fragments.

Types of Fc regions

The five different types of Fc regions allow antibodies to be grouped into five?isotypes. Each Fc region of a particular antibody isotype is able to bind to its specific Fc Receptor (except for IgD, which is essentially the BCR), thus allowing the antigen-antibody complex to mediate different roles depending on which FcR it binds. The ability of an antibody to bind to its corresponding FcR is further modulated by the structure of the glycan(s) present at conserved sites within its Fc region.?The ability of antibodies to bind to FcRs helps to direct the appropriate immune response for each different type of foreign object they encounter.?For example,?IgE?is responsible for an?allergic?response consisting of?mast cell?degranulation and?histamine?release. IgE's?Fab?paratope binds to allergic?antigen, for example?house dust mite?particles, while its Fc region binds to Fc receptor ε. The allergen-IgE-FcRε interaction mediates allergic signal transduction to induce conditions such as asthma.

Though the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures, or antigen-binding sites, to exist. This region is known as the?hypervariable region. Each of these variants can bind to a different antigen.?This enormous diversity of antibody paratopes on the antigen-binding fragments allows the immune system to recognize an equally wide variety of antigens.?

Antibody diversity.

The large and diverse population of antibody paratope is generated by random recombination events of a set of?gene?segments that encode different antigen-binding sites (or?paratopes), followed by random?mutations?in this area of the antibody gene, which create further diversity.?This recombination process that produces clonal antibody paratope diversity is called V (D) J or VJ recombination. Basically, the antibody paratope is polygenic, made up of three genes, V, D, and J. Each paratope locus is also polymorphic, such that during antibody production, one allele of V, one of D, and one of J is chosen. These gene segments are then joined together using random genetic recombination to produce the paratope. The regions where the genes are randomly recombined together is the hyper variable region used to recognize different antigens on a clonal basis.

Antibody genes also re-organize in a process called?class switching?that changes the one type of heavy chain Fc fragment to another, creating a different isotype of the antibody that retains the antigen-specific variable region. This allows a single antibody to be used by different types of Fc receptors, expressed on different parts of the immune system.

Forms of antibodies

The membrane-bound form of an antibody may be called a?surface immunoglobulin?(sIg) or a?membrane immunoglobulin?(mIg).

It is part of the?B cell receptor?(BCR), which allows a B cell to detect when a specific antigen is present in the body and triggers B cell activation.?The BCR is composed of surface-bound IgD or IgM antibodies and associated Ig-α and Ig-β?heterodimers, which are capable of?signal transduction.?A typical human B cell will have 50,000 to 100,000 antibodies bound to its surface.?Upon antigen binding, they cluster in large patches, which can exceed 1 micrometer in diameter, on lipid rafts that isolate the BCRs from most other cell signaling receptors.?These patches may improve the efficiency of the?cellular immune response.?In humans, the cell surface is bare around the B cell receptors for several hundred nanometers,?which further isolates the BCRs from competing influences.

Antibody–antigen interactions

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.

Often, once an antibody and antigen bind, they become an?immune complex, which functions as a unitary object and can act as an antigen in its own right, being countered by other antibodies. Similarly,?haptens?are small molecules that provoke no immune response by themselves, but once they bind to proteins, the resulting complex or hapten-carrier adduct?is antigenic.

Isotypes

Antibodies can come in different varieties known as?isotypes?or classes. In?placental?mammals there are five antibody isotypes known as IgA, IgD, IgE, IgG, and IgM. They are each named with an "Ig" prefix that stands for immunoglobulin, a name sometimes used interchangeably with antibody, and differ in their biological properties, functional locations and ability to deal with different antigens, as depicted in the table.?

The different suffixes of the antibody isotypes denote the different types of heavy chains the antibody contains, with each heavy chain class named alphabetically: α, γ, δ, ε, and μ. This gives rise to IgA, IgG, IgD, IgE, and IgM, respectively.

The antibody isotype of a?B cell?changes during cell?development?and activation. Immature B cells, which have never been exposed to an antigen, express only the IgM+ isotype in a cell surface bound form. The B lymphocyte, in this ready-to-respond form, is known as a "naive B lymphocyte." The naive B lymphocyte expresses both surface IgM+ and IgD+. The co-expression of both of these immunoglobulin isotypes renders the B cell ready to respond to antigen.?B cell activation follows engagement of the cell-bound antibody molecule with an antigen, causing the cell to divide and?differentiate?into an antibody-producing cell called a?plasma cell. In this activated form, the B cell starts to produce antibody in a?secreted?form rather than a?membrane-bound form. Some?daughter cells?of the activated B cells undergo?isotype switching, a mechanism that causes the production of antibodies to change from IgM or IgD to the other antibody isotypes, IgE, IgA, or IgG, that have defined roles in the immune system.

Structure.

Antibodies are heavy (~150?kDa)?globular?plasma proteins. They have sugar chains (glycan) added to conserved?amino acid?residues.?In other words, antibodies are glycoproteins.?The attached glycan are critically important to the structure and function of the antibody.?Among other things the expressed glycan can modulate an antibody's affinity for its corresponding FcR(s).

The basic functional unit of each antibody is an immunoglobulin (Ig)?monomer?(containing only one Ig unit); secreted antibodies can also be?dimeric?with two Ig units as with IgA, tetrameric?with four Ig units like?teleost fish?IgM, or?pentameric?with five Ig units, like mammalian IgM.

Several immunoglobulin domains make up the two heavy chains and the two light chains of an antibody. The immunoglobulin domains are composed of between 7 (for constant domains) and 9 (for variable domains) β-strands.

The variable parts of an antibody are its V regions, and the constant part is its C region.

Immunoglobulin domains

The Ig monomer is a "Y"-shaped molecule that consists of four?polypeptide?chains; two identical?heavy chains?and two identical?light chains?connected by?disulfide bonds.?Each chain is composed of?structural domains?called?immunoglobulin domains. These domains contain about 70–110?amino acids?and are classified into different categories (for example, variable or IgV, and constant or IgC) according to their size and function.?They have a characteristic?immunoglobulin fold?in which two?beta sheets?create a "sandwich" shape, held together by interactions between conserved?cysteines?and other charged amino acids.

Heavy chain

There are five types of mammalian Ig?heavy chain?denoted by the?Greek letters:?α,?δ,?ε,?γ, and?μ.?The type of heavy chain present defines the?class?of antibody; these chains are found in IgA, IgD, IgE, IgG, and IgM antibodies, respectively.?Distinct heavy chains differ in size and composition; α and γ contain approximately 450 amino acids, whereas μ and ε have approximately 550?amino acids.

Each heavy chain has two regions, the?constant region?and the?variable region. The constant region is identical in all antibodies of the same isotype, but differs in antibodies of different isotypes. Heavy chains γ, α and δ have a constant region composed of?three?tandem (in a line) Ig?domains, and a hinge region for added flexibility;?heavy chains μ and ε have a constant region composed of?four immunoglobulin domains.?The variable region of the heavy chain differs in antibodies produced by different B cells, but is the same for all antibodies produced by a single B cell or?B cell clone. The variable region of each heavy chain is approximately 110 amino acids long and is composed of a single Ig domain.

Light chain

In mammals there are two types of?immunoglobulin light chain, which are called?lambda?(λ) and?kappa?(κ).?A light chain has two successive domains: one constant domain and one variable domain. The approximate length of a light chain is 211 to 217 amino acids. Each antibody contains two light chains that are always identical; only one type of light chain, κ or λ, is present per antibody in mammals. Other types of light chains, such as the?iota?(ι) chain, are found in other?vertebrates?like sharks (Chondrichthyes) and bony fishes (Teleostei).

CDRs, Fv, Fab and Fc regions

Some parts of an antibody have the same functions. The arms of the Y, for example, contain the sites that can bind to antigens (in general, identical) and, therefore, recognize specific foreign objects. This region of the antibody is called the?Fab (fragment, antigen-binding) region. It is composed of one constant and one variable domain from each heavy and light chain of the antibody. The?paratopeis shaped at the?amino terminal end?of the antibody?monomer?by the variable domains from the heavy and light chains. The variable domain is also referred to as the FV?region and is the most important region for binding to antigens. To be specific, variable loops of β-strands, three each on the light (VL) and heavy (VH) chains are responsible for binding to the antigen. These loops are referred to as the?complementarity determining regions?(CDRs). In the framework of the?immune network theory, CDRs are also called idiotypes. According to immune network theory, the adaptive immune system is regulated by interactions between idiotypes.

The base of the Y plays a role in modulating immune cell activity. This region is called the?Fc (Fragment, crystallisable) region, and is composed of two heavy chains that contribute two or three constant domains depending on the class of the antibody.?Thus, the Fc region ensures that each antibody generates an appropriate immune response for a given antigen, by binding to a specific class of?Fc receptors, and other immune molecules, such as?complement?proteins. By doing this, it mediates different?physiological effects including recognition of?opsonized?particles (binding to FcγR),?lysis?of cells (binding to complement), and?degranulation?of?mast cells,?basophils, and?eosinophils?(binding to FcεR).

In summary, whilst the Fab region of the antibody determines its antigen specificity, the Fc region of the antibody determines the antibody's class effect. Since only the constant domains of the heavy chains make up the Fc region of an antibody, the classes of heavy chain in antibodies determine their class effects. Possible classes of heavy chains in antibodies include alpha, gamma, delta, epsilon, and mu, and they define the antibody's isotypes IgA, G, D, E, and M, respectively. This infers different isotypes of antibodies have different class effects due to their different Fc regions binding and activating different types of receptors. Possible class effects of antibodies include: Opsonisation, agglutination, hemolysis, complement activation, mast cell degranulation, and neutralization (though this class effect may be mediated by the Fab region rather than the Fc region). It also implies that Fab-mediated effects are directed at microbes or toxins, whilst Fc mediated effects are directed at effector cells or effector molecules.

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