Mast cells are powerful effector cells, responsible for a variety of immune responses, generally associated with inflammation. Mast cells release histamine and other agents of inflammation. They are crucial in responses to parasites and also primary agents in allergic reactions. They are also involved in immune tolerance, but this side of their role is less understood.
Mast cells arm themselves (as shown in the figure below) with IgE molecules, one class of antibodies. IgE are produced at the end of the antibody-switching sequence, and that causes them to be highly specific to antigens through a process of affinity maturation, that takes place when antibody production line proceeds through a sequence, generally IgM → IgG → IgA → IgE (there are also antibody subclasses, and other complications). The sequence does not always proceed in this fashion; there are jumps and skips, but it is a rule of thumb scheme. Antibody next in sequence has (usually) higher affinity to antigen than a preceding one. IgE molecules are produced in small quantities, compared to other classes of antibodies, but mast cells have IgE receptors that have VERY high affinity for IgE, thousands of times higher than usual affinity of cell receptors for IgG or IgA. IgE binding to mast cells is essentially irreversible (rather unusual for ligand-receptor interactions).
As a result, mast cell has antibodies that are ready to bind to an antigen, and can do that at VERY low antigen concentrations (because they have such high affinity). Consequently, mast cells can be activated by vanishingly small amounts of antigen, which is highly problematic for allergy sufferers.. For example, people who are hyper-reactive to fish, may start sneezing and wheezing by inhaling a whiff of fish aroma; others may get a dramatic response to a trace of peanuts, etc.
When antigen binds to IgE receptors, they begin to aggregate on the cell surface and cause the cell to degranulate. This means that the cell brings to cell membrane vesicles filled with biologically active substances, and these vesicles open to the outside. The researchers who described those cells in 19th century called these vesicles “granules” and hence the name of this event.
If a mast cell goes “nuclear”, it is like an incendiary bomb exploding in a neighborhood. The mediators of inflammation are themselves active, and they also induce other cells to release inflammatory agents. A tissue can be inflamed within seconds, with all kinds of physiological consequences – redness, swelling, narrowing of airways, etc.
Problems with allergy and asthma is that the very potent responses are generated in reaction to completely harmless, and the body tissues are damaged or even ruined by a response that is completely unneeded, and can be lethal. It is a profoundly misdirected response, and we do not have a good understanding of the reasons, although many factors can be potentially of importance. Pollution, chemical additives, and hormone disruptors created by breakdown of plastics is a good candidate. Very important is, most likely, a short duration of breastfeeding. Some people came up with a “cleanliness” hypothesis, posing that living in scrubbed environment can cause the immune system to become less tolerant of irritants. Some proposed that absence of intestinal parasites may contribute to allergies, as the immune system is left with little to do, and possibly begins to engage in semi-random responses to elements of environment.
Interestingly, there is evidence of synapses between the neurons and mast cells. It is possible to induce an asthma attack in a rabbit sensitized to some antigen through a Pavlovian conditioning. The animal is exposed to an aerosol with an allergen and the bell is rung. Once the response to allergen is in place, it can be induced by ringing a bell, without any presence of the allergen whatsoever. There is also some anatomical evidence for these synapses, and they may explain in part the effects of acupuncture.
Mast cells are vastly important, and their role in the development and functioning of the body is far from being understood, and probably underappreciated.