There are five types of immune proteins called immunoglobulins (Ig): IgG, IgD, IgA, IgM and IgE. In general, these immune proteins are an integral part of our host defense system. However, just as Benedict Arnold turned traitor to the American cause, so too our immune proteins can sometimes go awry. This is the definition of auto-immune disease, i.e. our own immune proteins attack us.
There are two ends of all five immune proteins and they are called the F ab end and the Fc end. F ab stands for “fraction that binds antigen”. It is the end of the protein that has specific target recognition capabilities. For instance, our Ig proteins we make from having been vaccinated for Tetanus have a very specific F ab end that only recognizes Tetanus, and thereby protects us from the disease by binding to it.
Fc stands for “fraction that is crystallizable” referring to how it was first discovered chemically – by crystallization techniques. But the Fc end of the molecule controls the immune function of the protein. The Ig exerts its effect when the Fc end of the protein fits into an Fc-receptor (much like an electric plug fitting into an outlet).
In the case of auto-immune diseases this has provided a mechanism to treat these diseases. To date, one of the most effective therapies for some auto-immune diseases is using intravenous immunoglobulin infusions (IVIG). This treatment has been a God-send in a number of auto-immune diseases such as Kawasaki disease, dermatomyositis, toxic epidermal necrolysis, immune thrombocytopenic purpura (ITP) and Guillain Barre syndrome to name a few. Basically, it works in part by flooding the body with Ig molecules containing Fc ends making it difficult for the abnormal Ig proteins to find an available Fc receptor. Think of the plastic devices used to “childproof” electrical outlets.
New on the horizon is the development of monoclonal antibodies to block Fc receptors. Currently there are seven
different monoclonal antibodies being developed for this purpose. So far, they seem to be both safe and effective, but
none have been developed long enough to be FDA approved. In addition to providing help for a whole variety of auto-immune diseases they may also find a very special niche in treating some fetal diseases during pregnancy. Of special research interest in this regard are hemolytic disease of the fetus, fetal alloimmune thrombocytopenia and anti-Ro disease which causes fetal heart block.