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Mast cell activation disease (MCAD)

Summary:

Mast cells are a particular cell type which belongs to the immune system and is found in all body tissues. Mast cells in the human body have as similar function like the watchtowers of an ancient city. The guards are looking for foreign intruders. Once they perceive something suspicious, they will alert the soldiers in their neighbourhood by using chemical signal substances. The soldiers then will rush to the location where the attack takes place to fend off the danger. Histamine is the most important of over 200 neurotransmitters that serve the communication with surrounding cells.

The disease occurs when some of these “guardians” become overzealous, hypersensitive or too numerous, causing frequent or permanent false alarms. The affected organs or the whole body are then unreasonably put into a "state of war", as if it was attacked by a pathogen. The body is feeling sick and reacting with numerous symptoms to the supposed, but non-existent threat.

Mast cell disorders are probably very common, but can not yet be reliably diagnosed. It is estimated that between 5 and 17% of the total population are affected.

What are mast cells?

Mast cells are also known as mastocytes. Mast cells got their strange name because their discoverer recognised under the microscope that, in contrast to other cells, they were stuffed with many small beads. He suspected that these cells had swallowed something, that they were fattened. (“Mast” means fattening of animals.) This later turned out to be a mistake. In the beads, messenger substances (chemical signalling substances) are stored, which serve the communication with other cells. The mast cell produces some of its messenger substances (mast cell mediators) in advance so that they can be rapidly released into the surrounding tissue if necessary. This cell type occurs as single cells in the entire body, especially along the outer borders (intestines, skin, mucous membranes) and along important organs (blood vessels, nerves).

The mast cells are formed from stem cells (precursor cells) in the bone marrow and then migrate through the body tissue until they settle somewhere to fulfil their function.

Normal function of mast cells

The mast cells are a certain type of cell belonging to the immune system. They can be described as guards and alarm centres. When they perceive possible threats to the body, they alert other cells by releasing certain messenger substances (biochemical signalling substances). Thanks to this communication between the cells, the surrounding tissue can be put into a state of emergency and certain immune cells are attracted.

The immune system, symbolically explained

How does the human immune system work? Imagine the human body being organized and structured like a city (see figure below). This city is a collection of many buildings (cells). Between the houses there are alleys (cell interspaces), as well as a public transport network consisting of buses (lymphatic system) and trams (bloodstream) as means of transport. At the outer borders, the city is bordered by a city wall (skin, mucous membranes), which serves as a protective barrier against external threats. There are watchtowers (mast cells) mainly along this city wall, but also inside the city. On these towers guards (receptors) look out for threats of all kinds. If the guards discover something foreign or dangerous, they trigger alarm signals (primary mast cell activation, release of chemical messengers). This alarm puts the neighbourhood in a state of emergency (local inflammation). The riot almost brings normal neighbourhood life to a standstill. Doors and windows are closed, the individual households adjust to a longer siege and prepare intensively for a house fight. Above all, the alarm signal alerts the soldiers and policemen of the affected district (various cells of the immune system) and directs them to the scene. They all run in the direction from which the alarm signal comes (chemotactic attracting of immune cells) in order to hopefully be able to eliminate the intruder with joint forces. The soldiers and policemen can mobilize further units if necessary (secondary mast cell activation), so that with large-scale operations after some time the whole city is put into alarm condition (feeling of illness in the entire body).

Mast cells can be imagined as the watchtowers in a city. These “guardian cells” look out for foreign invaders. As soon as they notice something suspicious, they alert the soldiers in their district to rush to the place of attack to render the danger harmless.

Mast cells have more than 200 different messenger substances at their disposal to communicate with their environment. The most important and well-known messenger substance that is released is histamine. Histamine is also released in large quantities in the case of allergies (recognition of a foreign body by means of antibodies). This substance then triggers certain processes in the body which serve as defence and lead to the known allergic symptoms (runny nose, reddened skin, itching, etc.). However, the mast cells do not only secrete messenger substances during the antigen-antibody-mediated recognition of an intruder. There are also other stimuli (chemical, mechanical, psychological) which are able to activate the mast cells.

Diseases of the mast cells

What goes wrong in the body with mast cell disease? If we imagine again that a human being is built like a city, then we can say that there are watchtowers (mast cells) that have become overzealous due to some pathological change. They constantly cause false alarms. They either permanently emit alarm signals, even if there is no danger at all, or they are hypersensitive, so that they sound the alarm even for small, harmless disturbances, or they multiply too strongly or no longer die, so that their number increases in certain districts (organs, tissues), which leads to more violent alarm signals. As a result, the affected neighbourhoods are in a much more frequent or permanent state of alarm. The alarm signals do not come from a specific location, as is usually the case, but from everywhere. The soldiers and policemen (cells of the immune system) no longer know if, when and where the body is attacked. The strength of the alarm signal causes the alarmed districts to alarm the neighboring districts as well, until finally the whole city mistakenly has the feeling that it is war. Hence the term “systemic mast cell activation disease” (systemic = system-wide, in the entire body or in the whole city respectively). The city dwellers take measures to avert danger (the body develops symptoms). These defensive measures can include, for example, inflammation of the tissues, the onset of a runny nose, the emptying of the intestines by means of diarrhoea, etc.

How often do mast cell diseases occur?

Mast cell diseases are just beginning to be discovered and in most cases cannot yet be reliably diagnosed. Therefore, it is not possible to give exact information about the frequency. Until recently, they were considered extremely rare. In the meantime, however, we are beginning to realise that this is a very common phenomenon. The estimates are constantly being revised upwards and are currently between 5 and 17% of the total population. The degree of severity can vary greatly from person to person, depends not only on physical causes but above all also on environmental factors, and even in every healthy person mast cells can be overactivated if the stimulus is strong enough. Therefore, it may not be possible to make a clear distinction between “affected” and “not affected”. The question is rather to what extent someone is affected.

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