An antibody switches its class due to rearrangements in the constant region of the heavy chain gene. Intervening DNA segments are removed and new ones are appended to the variable regions (see the figure entitled "Class Switching" below). The various constant region segments are given Greek letter names. The names of the antibodies that result use the English equivalents. As a result, there are five major classes to which an antibody can belong: IgM (constant region C-mu), IgG (constant region C-gamma), IgD (constant region C-delta), IgA (constant region C-alpha), and IgE (constant region C-epsilon). You may have noticed a discrepancy. There are nine constant regions in all, but only five classes. That's because several of the constant region segments come in subclasses. There are four C-gamma constant regions, numbered 1-4 and two C-alpha's. Each of these subclasses plays a slightly different role, a complication that's outside the scope of this discussion. Again, remember that the binding site of an antibody from any given B cell (and its descendants) remains the same regardless of the class of the constant region.
Antibodies start out by elaborating IgM's, the complicated pentamers pictured in the last posting. IgM antibodies are good at initiating the cascade of reactions that activate complement. I've already described how the innate immune system can activate the complement pathway. An alternative pathway to initiating complement activation is through IgM antibody binding. The initial steps are different, but the end result is the same: a complement cascade that can destroy invading microbes to which IgM has bound.
IgG antibodies represent a large fraction of all the antibodies in the blood. Their "Y" shape should be familiar to you by now. They're important in several ways. By binding to the outer walls of microbes, they mark pathogens for phagocytosis by cells of the innate immune system. Similarly, surface receptors on natural killer cells bind to them, marking those cells covered with antibody molecules for destruction. They also bind to viruses and some toxic products of bacteria, rendering them impotent. Like IgM's, IgG molecules can also spark the complement activation cascade, but they are less capable of doing so.
IgA antibodies are dimers - similar to two IgG monomers linked together. They are the most abundant of all antibodies - an average adult human makes two to three grams per day. Most of it is secreted into the gut where it acts to bind to pathogens. Apparently the structure of IgA makes it resistant to the degradative enzymes and harsh condition that are present there.
IgE antibodies resemble IgG molecules but, of course, have their own distinctive heavy chain tail. They are produced in response to a variety of parasitic infections and bind to their quarry in great numbers, cloaking it like snowflakes adhering to a parked car after a winter storm. Receptors to IgE antibodies on mast cells bind to the this surface coating. The newly recruited mast cells respond by releasing their toxic contents in the close vicinity of the parasite. In addition to the poisoning of their quarry, the result may be dilation of nearby blood vessels, contraction of smooth muscles, and fluid retention. These reactions are appropriate if some worm has infiltrated our body, but IgE is also expressed upon exposure to less harmful challengers. For example, pollen can elicit an IgE reaction and subsequent mast cell response. Our runny noses and watery eyes are evidence of the many pollens in the air interacting with our immune system. Another stimulant of IgE production is bee or wasp venom. Upon repeated stings, the body may respond massively, releasing mast cell contents in many tissues simultaneously. The result may be anaphylactic shock, a very dangerous condition.
Most activated B cells become plasma cells, highly productive workshops for synthesizing massive amounts of antibodies. They perform their duties with great energy, but when they have accomplished their job, when the invader has been vanquished, they die to be replaced by new B cells with different specificities. But by mechanisms unknown, a few B cells activated by helper T cells, undergo profound changes. They stop secreting antibody. They change their surface proteins. They remain alive, sometimes for many decades. With the help of T cells, they undergo further class switching and hypermutation, thereby becoming capable of secreting even more potent (and appropriate) antibodies. And they patrol the blood stream and lymph nodes, looking for some substance that matches their antigen recognition site. When they encounter such a molecule, they respond with force, much more powerfully then the initial antibody response.
Edward Jenner was the first to take advantage of this immunological memory in England the late 18th century. We continue to use more sophisticated versions of his technique, now called "immunization", to protect populations from a great variety of diseases.