- Antibodies are known as immunoglobulins (Igs), which is a group of soluble proteins.
- An antigen can cause the production of different antibodies if the antigen has several epitopes.
- Epitopes or antigenic determinants, are known as antigen binding sites.
- The number of antigen-binding sites is called the antibody’s valence.
- There are at least two antigen-binding sites on each antigen where a human antibody can bind. This is referred to as a bivalent antibody because the antibody’s valence value is two.
- The structure of a bivalent antibody is called a monomer and consists of four protein chains that are named after their relative molecular weights. These are two light (L) chains and two heavy (H) chains.
- These protein chains are joined together to form a Y-shaped molecule.
- There are two regions of the protein chain. These are the variable (V) region and the constant (C) region.
- The variable region is located at the ends of the arms of the Y.
- These are the sites where the antibody binds with an antigen.
- The variable region is a three dimensional structure of amino acid sequences whose structure reflects the epitopes of the antigen.
- The constant region is the stem of the Y and is called the Fc region.
- The Fc region binds adjacent complementary antibodies if both antigen-binding sites are attached to an antigen; otherwise the adjacent complementary antibodies are free to attach and react with antigen.
There are five types of constant regions each associated with the five classes of immunoglobulin
IgG, IgM, IgA, IgD, IgE
- IgG immunoglobulin neutralizes bacterial toxins and attacks circulating bacteria and viruses by enhancing the effectiveness of phyagocytic cells.
- 80 percent of the antibodies in serum are IgG.
- IgG can cross blood vessel walls and the placenta and can enter tissue fluids.
- IgM immunoglobulin is the first antibody to respond to an antigen or initial infection and makes up about 10 percent of antibodies in serum.
- IgM is relatively large in size and has a pentamer structure of five monomers bonded together by a joining (J) chain. This chain is a polypeptide. Because of its size.
- IgM remains in blood vessels and not in tissue fluids.
- IgM responds to the ABO blood group antigens and enhances the effectiveness of phyagocytic cells.
- When initial exposure to an antigen occurs, IgM antibodies are the first to appear.
- IgA immunoglobulin is the most common antibody in body secretions and in mucous membranes.
- It consists of about 15 percent of the antibodies in a serum.
- IgA protects infants from gastrointestinal infections and fights antigens that affect the respiratory tract.
- Plasma cells in the mucous membrane form secretory IgA, which is then passed through the mucosal cell and attacks antigens on the mucosal surface such as bacteria and viruses.
- IgA is short-lived.
- IgD immunoglobulin is found in blood and lymph fluid and is an antigen receptor on the surface of B cells.
- This also provides protection against parasitic worms.
- IgE immunoglobulin binds to basophil cells and mast cells that release chemical mediators, such as histamine, that cause an allergic reaction.
- When pollen is the antigen, the allergic reaction is referred to as hay fever.
- IgE is less than one percent of serum antibodies.
- This provides protection against parasitic worms.