A foreign macromolecule capable of generating an immune response (antibody formation is known as an antigen or immunogen. All cells of a human are characterized by the presena of a specific type of molecule on their surfaces. Another human’s cells have molecules of different configuration.
These molecules are called antigens. This is one of the bases of individuality Any molecule other than those found on the cell surfaces of an individual are considered as foreign and when, enters into the.system, evokes an immune response or antibody formation Another requirement is that an antigen must be a macromolecule or derived from| macromolecule (a virus particle).
The large molecule is not necessary for the antigenic property! The immune system responds to specific groups of atoms of the macromolecule. These specific groups are called antigenic determinants or epitopes.
A macromolecule may have two more antigenic determinants. Small foreign molecules do not stimulate antibody formation. The] do so when they are tagged to macromolecules. The small foreign by itself is called a happen a chemical group to which a hapten is attached is known as a haptenic determinant. Antibodie stimulated by an attached hapten will also bind to unattached haptens.