Q.Write notes on immunoglobulins.

Immunoglobulins are Y-shaped antibody proteins (two heavy + two light chains) made by plasma cells; the five classes are IgG, IgM, IgA, IgE and IgD, each with a special role.

Immunoglobulins (antibodies) are glycoproteins of the gamma-globulin fraction of blood, produced by plasma cells (activated B-lymphocytes) in response to antigens.

Basic structure:

• Each antibody molecule is Y-shaped and consists of four polypeptide chains: two identical long (heavy, H) chains and two identical short (light, L) chains, held by disulphide bonds. Hence an antibody is represented as H2L2.
• The tips of the Y form the variable antigen-binding sites, which are specific to a particular antigen.

The five classes of immunoglobulins:

1. IgG: The most abundant antibody in blood; provides long-term immunity and is the only class that crosses the placenta to protect the foetus.

2. IgM: The largest antibody (a pentamer); the first to be produced in a primary immune response; effective in agglutination.

3. IgA: Found in external secretions such as saliva, tears, sweat, mucus and colostrum/milk; guards mucosal surfaces.

4. IgE: Involved in allergic (hypersensitivity) reactions and defence against parasitic worms; binds to mast cells.

5. IgD: Present mainly on the surface of B-lymphocytes, acting as antigen receptors.

Immunoglobulins are the antibody proteins of humoral immunity, produced by plasma cells and grouped into five classes.

What they are: Immunoglobulins (Igs) are antibodies — glycoprotein molecules produced by plasma cells (differentiated, antigen-stimulated B-lymphocytes) as part of humoral (antibody-mediated) immunity. Each antibody is highly specific, recognising and binding to a particular antigen.

Basic structure: A typical immunoglobulin molecule is Y-shaped, built from four polypeptide chains — two identical heavy (H) chains and two identical light (L) chains — held together by disulfide bonds. Each chain has a variable region (which forms the antigen-binding site, differing between antibodies to give antigen specificity) and a constant region (which determines the antibody's class and effector function).

The five classes of immunoglobulins:

• IgG — the most abundant antibody in blood serum; the only class that can cross the placenta, providing passive immunity to the foetus/newborn.

• IgA — present in body secretions (saliva, tears, mucus, colostrum/breast milk); protects mucosal surfaces.

• IgM — the first antibody produced during a primary immune response; the largest immunoglobulin, existing as a pentamer.

• IgD — found mainly on the surface of B-lymphocytes, functioning as an antigen receptor that helps activate B cells.

• IgE — associated with allergic reactions (hypersensitivity) and defence against parasitic infections; binds to mast cells and basophils.