Antibody Interaction with Antigen

What is an antibody? An antibody is an immunoglobulin protein, secreted by B lymphocytes, that is present in serum or body fluid and combines specifically with an antigen. Antigens are classically defined as any foreign substance that elicits an immune response. An antigen that produces an adaptive immune response after injection into an animal is an immunogen. Immunogens can be designed so that antibodies are generated against specific proteins. Because antibodies can be made against specific proteins, they are very useful tools in science and can be used to investigate specific protein function and location in a dynamic biological system.

The specific region on an antigen that an antibody recognizes and binds to is called the epitope, or antigenic determinant. An epitope is usually 5-8 amino acids long on the surface of the protein. Proteins are three dimensionally folded structures, and an epitope may only be recognized in its form as it exists in solution, or its native form. When an epitope is made up of amino acids that are brought together by the three-dimensional structure, the epitope is conformational, or discontinuous. If the epitope exists on a single polypeptide chain, it is a continuous, or linear epitope. Depending on the epitope an antibody recognizes, it may bind only fragments or denatured segments of a protein, or it may also be able to bind the native protein.

Antibodies can be generated against specific peptide sequences or proteins by being conjugated to a carrier protein that is known to be strongly immunogenic. Common carrier proteins are keyhole limpet hemocyanin (KLH) and bovine serum albumin (BSA). When the immunizing peptide sequence or protein is conjugated to the carrier protein, the antibodies generated by the immunized animal will be specific to epitopes across the surface of the whole carrier complex.