Antigen-specific activation of B cells occurs following the binding of antigen to membrane-bound immunoglobulin. Under the influences of many different cytokines released from monocytes and T cells, B cells undergo clonal expansion and finally, differentiation into plasma cells capable of secreting large quantities of antibody. Little subsets of mature B cells turn out to be memory B cells, which are responsible for the recall responses following reexposure to antigen. When an individual first encounters a foreign antigen, an antibody response is mounted.
There are typically four stages that characterize the main immune response. During the first stage, no antibody is detected for the first 4 to 5 days. During the second phase, IgM antibodies form in high titers, followed a number of days later (usually 10 to 14 days after antigen exposure) by the production of IgG antibodies directed toward exactly the same antigen. In the third stage, the antibody titer stabilizes, and during the fourth stage, there is a decline in antibody titer over a period of months to years, as the antibody is either cleared or catabolized. The secondary antibody response occurs upon reexposure to the exact same antigen; antibody, primarily IgG, appears more quickly, persists longer, and reaches a greater titer.
Immunoglobulin structure. Immunoglobulins are glycoproteins composed of 4 polypeptide chains (two light and two heavy chains), linked by disulfide bonds that permit the chains to form a bilaterally symmetric immunoglobulin molecule. The N terminus of each chain possesses a variable domain, which, via the use of the hypervariable complementarity-determining regions, binds antigen. Both the heavy and light chains have C-terminal regions. These regions form the continuous regions and additional define the class and subclass of the antibody. This region also determines if the antibody light chain will be kappa or lambda.
There are 5 classes of immunoglobulins, known as IgA, IgG, IgM, IgD, and IgE, based on the structure of their heavy chains. There are two subclasses of IgA and four subclasses of IgG. All immunoglobulins are glycoproteins and contain in between 3% and 13% carbohydrate. The carbohydrate portion basically maintains the structure of the immunoglobulin. Each antibody can exist as two types: as a circulating molecule or as a stationary molecule attached towards the B-cell surface which functions as the B-cell receptor. The stationary form has a hydrophobic transmembrane portion that functions as an anchor.
You will find only two classes of light chains: kappa and lambda. Each immunoglobulin molecule has only one class of light chain and only one class of heavy chain, even though each class of immunoglobulin can have either kappa or lambda light chains. A monomeric antibody, such as IgG, consists of a single immunoglobulin molecule. Polymeric antibodies, like IgM and IgA, consist of multiple fundamental units.
In addition to the polypeptide chains, other structures can be incorporated into the immunoglobulin molecule, including the J chain, also known as the joining chain, which is associated with all polymeric forms of antibody and is really a polypeptide that stabilizes the polymer and the secretory piece of IgA. The secretory component of IgA can help to protect the IgA molecule from proteases within the gastrointestinal tract. This secretory component is only discovered on the secretory form of IgA.
Antibody diversity. To respond to the huge variety of antigens that humans along with other animals encounter, the immune system should be capable of producing roughly 1015 antigen-specific antibodies. The source of antibody diversity lies in the structure and arrangement of the immunoglobulin genes and also the ability of B cells to modify these genes by chromosomal rearrangement. You will find three genetic components that encode immunoglobulin. These 3 components are found on three separate chromosomes.
The antigen specificity of an antibody molecule depends upon the amino acid sequences in the antigen-binding portions of the heavy and light chain variable domains. The information needed for an immunoglobulin molecule to be produced is coded for in the DNA. The chromosomal light and heavy chain DNA is separated into multiple gene segments that code for the variable (V), joining (J), and continuous (C) regions of the light and heavy chains of the antibody molecule and also the diversity (D) region of the immunoglobulin heavy chain. The IGK and IGL clusters lack the D segment. The process of somatic mutation takes place in the germinal centers of secondary lymphoid tissue, during B-cell proliferation.
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