Mast cells are a type of immune cell found throughout the body, originating from myeloid stem cells in the bone marrow. They mature in various tissues, particularly in areas frequently exposed to the external environment, such as the skin, lungs, and gastrointestinal tract. These cells contain numerous granules rich in chemical messengers that play a role in the body’s defense system. Mast cells are involved in both protective responses against foreign invaders and certain inflammatory reactions.
The Body’s Alert System: Activation and Mediator Release
Mast cells function as an “alert system” for the body, responding to various stimuli by releasing chemical messengers called mediators. This process, known as degranulation, involves the rapid extrusion of granule contents into the extracellular space. They can be activated by pathogens, toxins, allergens, or even physical stimuli.
The release of mediators can be immediate for pre-stored substances like histamine, or delayed for newly synthesized compounds. Mediators stored in mast cell granules include histamine, proteases like tryptase and chymase, and heparin. Other mediators, such as cytokines, chemokines, and lipid mediators like prostaglandins and leukotrienes, are synthesized after activation.
Each type of mediator contributes to specific biological effects. For instance, histamine increases blood vessel permeability and can cause smooth muscle contraction. Proteases like tryptase are often used as markers of mast cell activation and contribute to inflammation. Cytokines and chemokines help recruit other immune cells to the site of activation, coordinating a broader immune response.
Mast Cells and Allergic Reactions
Mast cells are central to allergic responses, particularly Type I hypersensitivity reactions, which include common allergies and anaphylaxis. In these reactions, allergens bind to specific IgE antibodies already attached to IgE receptors on the mast cell surface. This binding triggers the mast cell to degranulate and release its mediators.
The rapid release of histamine from mast cell granules is responsible for many characteristic allergy symptoms. Histamine causes vasodilation, leading to redness and swelling, and increases vascular permeability. It also stimulates nerve endings, causing itching.
In the respiratory system, histamine can induce bronchoconstriction, leading to difficulty breathing, a common symptom in asthma and severe allergic reactions. In the gastrointestinal tract, mast cell mediators can cause symptoms like nausea, vomiting, and diarrhea. Anaphylaxis, a severe, potentially life-threatening allergic reaction, involves widespread mast cell activation and mediator release.
Beyond Allergies: Diverse Roles in Immunity and Health
Beyond their well-known role in allergies, mast cells participate in diverse physiological and immune functions. They are involved in wound healing, where they contribute to acute inflammation, promote the growth of new blood vessels (angiogenesis), and stimulate tissue remodeling. Mast cells secrete growth factors and cytokines that support the proliferation and migration of cells involved in repair.
Mast cells also play a role in defending against pathogens, including parasites and bacteria. They can recognize harmful antigens directly or through specific receptors like Toll-like receptors, leading to the release of antimicrobial mediators. This action helps to eliminate pathogens and recruit other immune cells like neutrophils and eosinophils to the site of infection.
Mast cells influence the functions of other immune cells, including dendritic cells, macrophages, T cells, and B cells, through the release of cytokines and chemokines. They contribute to maintaining gut homeostasis and are involved in detoxifying certain venoms and endogenous toxins.
When Mast Cells Become a Problem
When mast cell function becomes dysregulated or excessive, it can lead to various health issues. Conditions like mastocytosis and mast cell activation syndrome involve abnormal mast cell behavior. Mastocytosis is characterized by the excessive proliferation of mast cells, which infiltrate various tissues and organs.
Mast cell activation syndrome involves inappropriate and increased activation of mast cells, leading to excessive mediator release without necessarily having an increased number of mast cells. Both conditions can result in a wide range of symptoms due to the widespread effects of mast cell mediators. Symptoms can include skin reactions like flushing, itching, and hives, as well as gastrointestinal issues such as abdominal pain, nausea, and diarrhea.
Other symptoms can affect the cardiovascular system, leading to rapid heart rate or low blood pressure, and the nervous system, causing headaches, brain fog, or anxiety. These symptoms can occur in episodes, often mimicking allergic reactions or even anaphylaxis.