The immunoglobulins, which are a collection of different types of molecules in the body, are an essential part of the immune system. Another name for them is antibodies. Each type has a specific designation by letter, and immunoglobulin D (IgD) makes up only a tiny portion of all the immunoglobulins. As of 2011, its exact function is not known, although it does appear to have regulatory function.
Antibodies are molecules inside the body that recognize dangerous invading foreign material and target it for destruction by the rest of the immune system. The main five types of antibodies are referred to by letter. Immunoglobulin G (IgG) is the most common, followed by IgA and then IgM. Combined, IgE and IgD make up only a small proportion of the antibodies in the body.
Immunoglobulin D was first noticed in the mid 1960s, but as of 2011, its function is not yet known. All of the other immunoglobulins latch onto foreign invaders and mark them for the immune system to destroy, so immunoglobulin D is assumed to perform a similar function. Scientists know that some IgD molecules move around freely in the blood system, whereas others are stuck onto particular immune system cells.
B cells are those cells of the immune system that either produce new antibodies in response to an infection, or turn into special memory cells that retain the recognition for a particular foreign invader for future reference if necessary. Immunoglobulin D is often found stuck onto the outside of B cells. Usually the B cell also has IgM molecules stuck on as well as IgD.
Researchers know that the immune system is complex, and lots of components interact with each other. A change in the level of one component can alter the level of another, or activate another component, and so on. Immunoglobulins are able to tell immune system cells to do certain things in certain conditions, and IgD is understood to be able to do this too.
Scientists believe that IgD can signal the body to make more of specific molecules, such as tumor necrosis factor alpha, which plays a role in inflammation and cell death. IgD may also activate B cells in the presence of infection and boost the efficiency of other immunoglobulins under certain conditions. Its presence can also influence other cells to produce chemicals.
Despite the theoretical uses of immunoglobulin D in the body, as understood by scientists, the full picture of how the molecule works is not yet clear. Doctors may still test patients for IgD levels, though, as some medical problems show an increased, or reduced concentration of IgD in the blood as a symptom. Tumors of the bone marrow, such as myeloma, are especially identifiable through IgD concentrations, among other medical signs.