Antibodies, also called immunoglobulin, are protective, non-enzymatic proteins produced by the immune system in response to the presence of antigens.
Antibodies are non-enzymatic proteins found in the immune system. Antibodies are large proteins produced by B-cells that are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses. The antibody recognizes a unique part of the foreign target know as an antigen.
Each tip of the antibody contains a paratope that is specific for one particular epitope (analogous to a lock and key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can tag a microbe or an infected cell for destruction by the immune system.
The general structure of all antibodies is very similar. The Ig monomer is a Y-shaped molecule that consists of four polypeptide chains: two identical heavy chains, and two identical light chains connected by disulfide bonds. Each chain is composed of structural domains called immunoglobulin domains. The constant region determines the class of an immunoglobulin. All chains have a characteristic immunoglobulin fold in which two beta sheets create a “sandwich” shape, held together by interactions between conserved cysteines and other charged amino acids.
However, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures, or antigen-binding sites, to exist. This region is known as the hypervariable region. Each of these variants can bind to a different antigen. This enormous diversity of antibodies allows the immune system to recognize an equally wide variety of antigens, allowing for a key feature of the adaptive (or acquired) immune system. The paratope is shaped at the amino-terminal end of the antibody monomer by the variable domains from the heavy and light chains.
Practice Questions
Khan Academy
MCAT Official Prep (AAMC)
Official Guide B/B Section Question 28
Key Points
• An antibody is a large non-enzymatic protein produced used by the immune system to identify and recognizes a unique part of the foreign target, called an antigen.
• Each tip of the “Y” of an antibody contains a paratope that is specific for one particular epitope (analogous to a lock and key) on an antigen, allowing these two structures to bind together.
Key Terms
Antibody: Non-enzymatic protein found in the immune system.
Antigen: A substance that binds to a specific antibody; may cause an immune response.
Non-enzymatic proteins: proteins that carry out functions that require the capacity to bind, but do not necessarily catalyze a reaction
Paratope: That part of the molecule of an antibody that binds to an antigen
Epitope: the part of an antigen molecule to which an antibody attaches itself.