Immunoglobulin G

Immunoglobulin G ( IgG) or gamma globulin are the antibodies (immunoglobulins ), class G, which act primarily against viruses and bacteria. The formation of immunoglobulins is part of the humoral immune response, ie, the non- cell-bound immunity by soluble substances in the blood. Biochemically are glycoproteins. They are produced by B- lymphocytes or plasma cells, after contact with an antigen.

Newborns can initially do not form their own IgG and are temporarily dependent on the mother's antibodies. Already in fetal blood can be detected IgG. It crosses the placenta with the help of specific transport receptors.

Structure

The antibody is composed of two long ( "heavy" ) and two short ( "light" ) chains and protein is in the form of a "Y". The heavy chains have a molecular mass of 50,000 daltons and are always of the class gamma to you, the light chains have a molecular weight of 22,000. The entire Molar mass is approximately 144,000 daltons ( 2 × 2 × 50,000 22,000 ). , The binding sites on the antigens (foreign body, for example, specific surface structures of bacterial cells ), or haptens are capable of binding to the short ends of the Y. The antigen-binding portion of IgG (Fab fragment) is highly variable. It has been calculated that approximately 106 to 109 chemically distinct Fab portions are possible. The corresponding gene segments subject to the corresponding necessary mutations during life. The binding to surface antigens triggers further immunological reactions that can lead to the destruction of the affected cells or antigen -presenting structures.

The heavy chains of the immunoglobulins of class G (gamma ) are divided into four isotypic subclasses (gamma 1-4). The light chains are found (kappa and lambda) in two ways.

By specific enzymes (e.g., papain or pepsin ) may be split into two types of the IgG antibody fragment. The three fragments obtained after cutting with papain, are:

• Fab Two fragments containing a monovalent antigen-binding site
• Fc: A fragment capable of binding to complement proteins or cellular Fc receptors

If you cut the immunoglobulin with pepsin, then we obtain:

• F [ab] 2: A fragment containing a divalent antigen-binding site, and held together by the disulfide bonds of the hinge region
• Fc: There will be no Fc fragment, as the heterodimer will no longer held together by the disulfide bonds of the hinge region. The result are two polypeptide chains

When cutting the immunoglobulin with plasmin yields:

• FACB: A fragment containing a divalent antigen-binding site ( paratope ), and further held together by the disulfide bonds in the polysaccharide and the Fc portion is
• PFC: two polypeptide chains that are held together by non- bonds.

Function

The immunoglobulins itself can not destroy the corresponding target, rather they have the task to highlight the objectives and available to other defense systems more vulnerable. However, since an IgG can bind to only one antigen, a part of the formation of antibodies to the specific part of the immune system. Certain plasma cells form specific antibodies that are directed against a particular antigen. One therefore speaks of the function of the plasma cell as a memory cell. However, since the mass of antibodies by plasma cells in the context of infection must be preceded by a previous phase of sensitization with the antigen, this strong antibody formation is called the secondary immune response. As part of a vaccination antibody formation against a particular antigen can be " learned " by the plasma cells and may enable the organism in a position to form upon renewed contact with the same antigen, even after many years rapidly and in large numbers, specifically acting antibodies.

In the spleen, the immunoglobulins are partially disassembled, with the tissue hormone tuftsin ( a tetrapeptide ) is released.

Pathology

Endogenous antigens are considered to be " self" in the context of self-tolerance and do not usually lead to a specific antibody response against these antigens. In autoimmune diseases, however, it may come to the formation of antibodies against the body's own structures, which have a pathological significance.

Disorders of the antibody IgG formation are called gammopathies. The hypogammaglobulinemia indicates a pathologically to low production of IgG antibodies. The agammaglobulinemia means the failure of antibody formation. Severe protein deficiency ( inanition, long periods of starvation ) or severe kidney problems ( nephrotic syndrome ) can affect the formation of antibodies. A reduction in the IgG formation leads to some serious infectious diseases.

Laboratory

The immunoglobulins G are clearly visible in the electrophoresis in the range gamma 1-2 as a separate peak. In the immunoelectrophoresis they are also identifiable. In human blood, about 8 grams of IgG per liter are included and represent 11-18 % of the total protein in the blood dar.

Immunoglobulin G | immunoglobulin M | immunoglobulin A | Immunoglobulin D | Immunoglobulin E