Antibodies are the proteins which are found in the human body. They are also called immunoglobulins. Thus, they are abbreviated as Igs. Igs are gamma globulin part of blood proteins.
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The structure of antibody can be described as:
- They are made of 4 looping polypeptide chains linked together by disulfide bonds.
- 2 chains are identical and are called heavy (H) chains and are made up of about 400 amino acids each.
- The other 2 are also identical to each other and are called light (L) chains but are only about half the size of the heavy chains.
- Hinge region is at the approximate middle where two disulfide bonds connect the two heavy chains.
- All four chains together form a molecule called an antibody monomer and is roughly T or Y shaped.
- Variable region are at the ends of the H and L chains and together form the antigen binding site.
- Constant regions form the stem of the monomer and determine the antibody class.
- Constant regions are the effector regions and dictate which cells and chemicals the antibody can bind to and how the antibody class will function in antigen elimination.
There are 5 classes of antibodies. They can be described as:
- IgD – It exists as a monomer and is attached to the external surface of the B cell where it functions as an antigen receptor and is important in B cell activation.
- IgM – exists in a monomer or pentamer form.
- monomer attached to the B cell surface and is an antigen receptor.
- pentamer circulates in blood plasma and is released by plasma cells during the primary response. It presence in the blood indicates current infection and it acts as a potent agglutinating agent and readily fixes and activates complement.
- IgG – It exists as a monomer and is the most abundant and diverse antibody in plasma and accounts for 75-85% of circulating antibodies. Can protect against bacteria, viruses, and toxins and can fix complement. It is the main antibody of both primary and secondary responses.-
- IgA – It is a dimer in plasma and is also called secretory IgA because it is present in body secretions such as saliva, sweat, intestinal juice, and milk and helps prevent pathogens from gaining access to the body.
- IgE – It is a monomer that is secreted by epithelial plasma cells and almost never found in the blood. They bind to mast cells and basophils when activated by antigen and it causes those cells to release histamine and other chemicals that mediate inflammation and allergic reactions. Blood IgE levels rise dramatically during allergic reactions or chronic gastrointestinal tract parasites.
Q.1. Antibodies are also known as:
Q.2. Name the bond present in the structure of antibodies:
- Amide bond
- H bond
- Covalent bond
- Disulfide bond
Q.3. Which antibody is also known as secretory immunoglobulin:
Q.4. How many polypeptide chains are present in the structure of a antibody:
Q.5. Which antibody is present in monomer and pentamer form:
Q.6. Which antibody is associated with the mast cells?