Antibodies are also called immunoglobulins.
Antibodies, also called immunoglobulins, are proteins that constitute a vital part of the body's immune response to antigens. Technically, any foreign substance that triggers an immune response may be called an antigen. Common examples include pollen and bacteria, as well as blood received during a transfusion or tissue from a transplant. Their presence in the body may cause the immune system's B-lymphocytes, or B-cells, to produce antibodies. These ends of the antibodies bind to the antigen, marking it for destruction. Some kinds of antibodies can also bind to placental cells to provide a developing fetus with passive immunity.
There are millions of B-cells, each of which is capable of producing an antibody designed to fight off a specific invader. In fact, most types of antibodies are capable of binding with only one antigen, though some antibodies can bind to more than one type if they are closely related. The process of binding is very specific, following a kind of method that depends on a number of complicated factors, including hydrogen bonds, hydrophobic interactions, van der Waals forces and electrostatic forces.
According to the University of Arizona, all antibodies are made up of four polypeptide chains, two of which are light and two of which are heavy. The chains compose a Y-shaped protein that has both a constant and variable region. The variable region is the site that binds the antigen, located at the ends of the Y's arms. The constant region is composed of the heavy chains and stretches down to the base of the Y. It determines how the antibody fights off the antigen. The hinge region is the spot where the Y branches out, and is so called because the molecule is flexible at this point.
The constant region is used to define the class of the antibody. As reported by the University of South Carolina, there are five major types of heavy chains, each of which is associated with a different set of clinical conditions:
Gamma heavy chains, IgG
Mu heavy chains, IgM
Alpha heavy chains, IgA
Delta heavy chains, IgD
Epsilon heavy chains, IgE
According to Emory University, a B-cell will start to reproduce once it comes across an antigen so that enough antibodies can be made to fight it off. Usually one antigen will trigger this response in numerous B-cells, each of which produces an antibody that can effectively attack a different portion of the invader. Polyclonal is the word used to describe a mix of antibodies, as opposed to a monoclonal sample of one antibody type produced in a lab.
Dalhousie University points out that a vast majority of B-cells, some 90 percent, never encounter an antibody and die within a few days. B-cells that have not encountered an antigen are said to be "nave," but those that do fight off a foreign substance undergo a process of activation to become B memory cells. These cells produce antibodies with powerful binding sites, so they react to antigens more rapidly than nave B-cells. B memory cells are found in the spleen, lymph nodes and bone marrow.