Immunoglobulins

Immunoglobulins, also known as antibodies, are glycoprotein molecules produced by plasma cells (white blood cells). They act as a critical part of the immune response by specifically recognizing and binding to particular antigens, such as bacteria or viruses, and aiding in their destruction. The antibody immune response is highly complex and exceedingly specific. The various immunoglobulin classes and subclasses (isotypes) differ in their biological features, structure, target specificity and distribution. Hence, the assessment of the immunoglobulin isotype can provide useful insight into complex humoral immune response. Assessment and knowledge of immunoglobulin structure and classes is also important for selection and preparation of antibodies as tools for immunoassays and other detection applications.

Immunoglobulin classes (isotypes): The various antibodies produced by plasma cells are classified by isotype, each of which differs in function and antigen responses primarily due to structure variability. Five major antibody classes have been identified in placental mammals: IgA, IgD, IgE, IgG and IgM. This classification is based on differences in amino acid sequence in the constant region (Fc) of the antibody heavy chains. IgG and IgA are further grouped into subclasses (e.g., in human IgG1, IgG2, IgG3, IgG4, IgA1 and IgA2) based on additional small differences in the amino acid heavy chain sequences.

Based on differences in the amino acid sequence in the constant region of the light chain, immunoglobulins can be further sub-classified by determination of the type of light chain (kappa light chain or lambda light chain). A light chain has two successive domains: one constant domain and one variable domain. The ratio of these two light chains differs greatly among species, but the light chains are always either both kappa or both lambda, never one of each.

Determination of individual subclasses is relevant in assessing primary immunodeficiencies or immune responses, especially if the total IgG or IgA concentration is not altered or elevated.

Isotype class switching: One of the primary functions of B cells in adaptive immunity is that of effecting a humoral response through the secretion of specific antibodies to address invading bodies and their toxic products. Some of these cells can undergo a “class switch” that causes expression of a new antibody isotype. For example, the antibody isotype could switch from an IgM to an antibody of all possible classes (e.g., IgG1,...IgG4, IgE). During this switch, the constant region of the heavy chain is changed, but not the variable region of the heavy chain. This switch does not affect the antibody’s specificity for its antigen, but it does alter the effector functions that each class of antibody can execute. The antibody class switch is critically dependent on the type of cytokine that is present. Various cytokines, such as IL-4, IL-5, IFN-gamma and TGF-beta, are known to be responsible for class switching. At a certain stage, the cell will lose its ability to undergo a switch to a class that has been generated before.

The Immunoglobulin, is a large Y-shaped protein produced by B- cells and used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. Five isotypes of antibodies are found in different locations and perform different specific functions.

• Opsonization- They bind to the surface of immunogens and the Fc region interacts with the phagocytes ("calls" them to the site of infection)
• Neutralization- They stick to antigens and block their attachment sites. ...
• Agglutination.
• Antibody mediated cytotoxicity.
• Complement activation.

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