B cells
The main function of B cells is to produce antibodies. Antibodies are complex molecules produced by the immune system in response to antigens. As mentioned previously, antigens are foreign proteins or glycoproteins (a sugar linked to a protein) that trigger the immune response. Every living cell produces several different proteins, each unique to its own cell type and species. The antibody produced against the antigen is entirely specific to that antigen. The two fit together like a lock and key, forming an antigen-antibody complex. When the body is invaded by a microorganism such as a bacterium or virus, B cells will make an antibody precisely designed to couple with molecules of that specific microorganism and no other.
The molecules of the microorganism that trigger that allergic response mediators are antigens. Similarly, if a pollen or a food causes B cells to make an antibody in the allergic response, the molecules of the pollen or food causing antibody production are antigens. When we talk about allergy, we call the antigens allergens. In the process of making antibodies, B cells first convert into plasma cells. The antibody is then generated from plasma cells.
The antibodies produced by B cells are of five different types, or classes, called IgA, IgG, IgM, IgE, and IgD. The Ig stands for immunoglobulin. Each antibody molecule is made up of a special protein called a globulin. Because the globulin is associated with immune system, the prefix immuno- is attached to it. Each of the five antibodies (IgA, IgG, IgM, IgE, and IgD) has a very specific role in immune protection and in the reactions that are responsible for adverse reactions to drugs or foods.
IgM
IgM is the largest of the antibodies and is found circulating in blood. It acts by seeking out antigens and attaching them to an end of one or more of its five “arms.” It has ten attachment sites (two per arm) and can mop up many antigens at a time. IgM is the first line of defense against a foreign molecule when it reaches the blood stream. As we shall see later, IgM is the first protective anti-body to be produced by the fetus in the womb (in utero).
IgA
IgA is found mainly in mucous secretions (secretions from surfaces exposed to the outside world through orifices such as the mouth, respiratory tract, vagina, etc.), where it is called secretory IgA (sIgA) to distinguish it from the IgA found in blood (usually called serum IgA). sIgA acts as the first line of defense against foreign molecules entering through external orifices, before they can enter the bloodstream. sIgA is extremely important in defending the digestive tract of the newborn baby against anything entering through its mouth. However, as we shall discuss later, the newborn baby (neonate) has no sIgA of its own, and its early digestive tract protection comes entirely from its mother’s colostrum—the first fluid that passes through the breast before mature milk is produced—which contains an abundant supply of sIgA.
IgG
IgG is the most important antibody in the immune system’s defense against invading disease-causing microorganisms. It is found in the bloodstream after the first-line IgM has started to mop up the invader. In effect, B cells first produce IgM and then switch over to IgG once it has been established that the invader is a real threat to the body. Even after the disease has been successfully suppressed, IgG remains to ensure that the same microorganism is “neutralized” when it enters the body on a subsequent occasion. IgG is the resident guardian that looks for a known dangerous invader that can cause the same infection again. This same process occurs when you receive an immunization shot that contains killed or “attenuated” microorganisms and defends the body against the real live virus infection or bacterium. We shall be looking at the role of IgG in reactions to foods in some detail, so a little more information about IgG is important. There are four different types of IgG antibodies; each type functions in different situations. These four types are called IgG1, IgG2, IgG3, and IgG4. In general, IgG4 antibodies are important in certain types of immunological (or hypersensitivity) reactions against foods. IgG1, IgG2, and IgG3 are important in the defense against invading pathogens and other agents that can cause disease in the body.
IgE
IgE is the most important antibody in allergy of all types, including the “classic allergy” of hay fever, asthma, skin reactions such as hives and angioedema (tissue swelling), and the potentially life-threatening anaphylactic reactions. The only other role for IgE, apart from the allergic reaction, is in fighting parasites and intestinal worms (helminths and nematodes).
IgD
The role of IgD is less well defined and is usually associated with aiding other immune functions such as “switching” from one antibody type to another. It is mentioned here simply for completeness. Its role in allergy is probably minimal.
