Closer Look at Binding

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 that is flexible enough to form a T-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 epi-topes 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.

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