Immunoglobulin D, also called IgD, is an uncommon antibody that is produced by immature B cells. The exact purpose of this antibody is unknown, though it has something to do with the humoral immune response in humans and other animals. It is similar in function to immunoglobulin M, which is also produced by B cells.
Like many other immunoglobulins, IgD is made up of four chains of amino acids. Two heavy and two light chains are connected by a hinge in the middle. The molecule is bent in such a way that three separate branches come off of the central hinge, which in this particular immunoglobulin is particularly long. This makes it susceptible to damage from proteases that can dissolve the proteins in the molecule and break it apart.
Another antibody with the same function is immunoglobulin M (IgM). Together, these two molecules are responsible for activating B cells when antigens are introduced into the body. Once the B cells are active, they attempt to destroy the antigen by producing other types of immunoglobulins.
IgM is present on younger B cells and is replaced by IgD as the B cell matures. There does not seem to be any difference in the function of either of these molecules. IgD is found on the surface of B cells as well as in the blood serum, though the amount in the blood serum is extremely low. Many human subjects have undetectable levels of this antibody, but this does not appear to affect overall immune function.
IgD is also used to instruct other immune cells, including mast cells and basophils, to begin producing antimicrobials. These compounds help protect the respiratory system from foreign bodies that can enter through the lungs. Scientists believe that there are other purposes for the antibody IgD, though the nature of these other functions is still unknown.
Though it is not the rarest form of immunoglobulin, IgD only makes up about 0.25% of the total amount of immunoglobulin found in the blood serum. The long hinge makes it easily broken down, which means that this molecule does not usually last very long. Approximately 37% of IgD in the blood stream is destroyed and resynthesized each day, and the half-life of this molecule is only about 3 days. The body is constantly creating more of these immunoglobulins in order to replenish what has been destroyed.