A foreign macromolecule capable of generating an immune response (antibody formation) is known as an antigen or immunogenic.
All cells of a human being are characterized by the presence of a specific type of molecule on their surfaces. In an immunological jargon, these molecules are sometimes referred to as molecular identity cards or molecular passports. These molecules are also termed as self-molecules.
The cells of another individual have molecules of a different configuration. They do not match with those found on the surface of another individual’s cell in respect of their configurations. These molecules, which assist in establishing the identity of cells of an individual, are called antigens.
This is one of the biological bases of individuality. Any molecule other than those found on the cell surfaces of an individual are considered as foreign and when, this enters into the system, evokes an immune response or antibody formation.
Following their formation, the antibodies combine with the antigens in a species-specific manner and pave the way for their elimination from the body. Besides being foreign molecule, another requirement is that an antigen must be a macromolecule or derived from a macromolecule (a virus particle). The large molecule itself is not necessary for its antigenic property.
The immune system responds to specific groups of atoms of the macromolecule. These specific groups are called antigenic determinants or epitomes. Thus an antigen may have two or more antigenic determinants. A specific type of antibody is synthesized in response to an antigenic determinant.
An antigen with many antigenic determinants will direct the synthesis of as many types of antibodies. Small foreign molecules do not stimulate antibody formation. They do so when they are tagged to macromolecules. The small foreign molecule by itself is called a happen and the chemical group to which a happen is attached is known as a herpetic determinant. Antibodies stimulated by an attached happen will also bind to unattached happens.
Interaction of an antibody with its antigenic determinant:
Antibodies are highly specific to their antigenic determinants, with which they combine in a species-specific manner. The part of the antibody, which confers specificity, lies in the variable region of both the light and heavy chains (VL and VH). The amino acid sequence in the variable region varies from antibody to antibody species. Further, there is a stretch of amino acids in the variable region of the antibody molecule, known as hypervariabe region (HVR). This region has three amino acid stretches, whose sequences differ to a very great extent from antibody to antibody. These stretches are known as complementarily determining regions (Cars).
These regions are actually vested with the property of specificity to an antigenic determinant. These three regions on both chains together form an antibody-binding site (Fib). A complete antibody molecule is a symmetrical four-chain unit (monomer) of two each of light and heavy chains. Therefore, there are two antigen-binding sites (Fab2) in a monomer of the antibody molecule. However, immunoglobulin A (Riga) is a dimmer and immunoglobulin M (IBM) is a pen tamer. Accordingly, they have four and ten antibody binding sites, respectively.
As discussed above, an antigen has many active groups of atoms known as antigenic determinants. Each determinant triggers the formation of a specific type of antibody and binds to the binding site of its antibody.
Binding occurs in a reversible manner by the formation of non- covalent bonds between the amino acids of the binding site and the functional groups of the antigenic determinant. The multiplicity of these bonds increases the binding strength. The strength of a single antigenic determinant-antibody binding is expressed as antigen affinity.
All antibodies are potentially bivalent or multivalent. For example, an Gig has two binding sites (divalent), Riga has four (tetravalent) and IBM ten (multivalent). The strength with which a multivalent antibody binds to multiple antigenic determinants is known as avidity. It is the summation of individual affinities.