Mast cell

Mast cells are specialized white blood cells that act as sentinels for the body.

They belong to the myeloid lineage and function within both the immune and neuroimmune systems. These cells reside in connective and mucosal tissues, often surrounding blood vessels, nerves, and lymphatic vessels. They are essential for detecting signals from parasites, pathogens, and other potential dangers. By managing inflammation and tissue repair, mast cells help maintain a healthy state. They play vital roles in wound healing, angiogenesis, and responding to bacteria, viruses, and fungi.

To perform their duties, mast cells use small secretory granules.

These granules store chemical mediators like histamine and heparin. When a mast cell detects a threat, it can release these substances in two ways. The first is piecemeal degranulation, which is a selective release of specific compounds. The second is anaphylactic degranulation, which is a rapid, large-scale release of mediators. Mast cells can also secrete newly synthesized mediators to manage long-term responses. This process allows the body to coordinate complex immune responses to various stimuli.

In humans, mast cells are categorized into three main types based on their protease expression.

The first type is MCT, which primarily resides in the mucosa of the lungs and small intestine. In fact, approximately 98% of mast cells in the human small intestine are MCT. The second type is MCTC, which is found in the skin, lymph nodes, and the submucosa of the gut. MCTC expresses tryptase, chymase, and carboxypeptidase. The third type is MCC, which expresses chymase but does not contain tryptase. These variations allow cells to be specialized for their specific environments.

In rodent models, the classification is slightly different. Researchers identify two major categories: connective tissue-resident mast cells (CTMCs) and mucosal mast cells (MMCs). CTMCs contain heparin and high amounts of histamine and carboxypeptidase. They are distributed in the skin and the peritoneal cavity. MMCs predominantly contain chondroitin sulfate and are found in the gastrointestinal tract. These different chemical profiles help the cells react appropriately to the specific threats found in different body locations.

The history of mast cell study began in the 19th century. Friedrich von Recklinghausen first described these cells in 1863. Later, in 1877, Paul Ehrlich rediscovered them and provided the name we use today. We now know that mast cells are among the most ancient immune cells. They likely originated nearly 500 million years ago in urochordates. This long history shows how fundamental these cells are to animal life.

The development of mast cells is a complex, multi-wave process. In humans, the first mast cells originate from mesodermal precursors in the yolk sac. This begins around three weeks into gestation. These progenitors then migrate into peripheral tissues to complete their maturation. While there is debate about whether adult mast cells come from bone marrow, many fetal-derived cells persist into adulthood. These cells often maintain themselves independently of the bone marrow.

Mast cell activity can be seen in common physical reactions. For example, a mosquito bite triggers a "flare and wheal" reaction. When the cell detects an allergen, it releases histamine. Histamine dilates blood vessels and increases permeability, causing redness, warmth, and swelling. It also depolarizes nerve endings, which causes itching or pain. While this helps fight threats, mast cell dysfunction is also linked to allergies, anaphylaxis, and atopic dermatitis.