Mast cells are a type of immune cell derived from the myeloid lineage. They are present in connective tissues throughout the body and are responsible for immediate allergic reactions by releasing products called "mediators". These mediators can cause allergic symptoms such as itching, swelling, and hives. Mast cells can also be activated by other substances such as medications, infections, insect or reptile venoms, and abnormal internal signals. Mast cells are oval or irregularly shaped cells with a dense granular cytoplasm that often obscures the nucleus and other organelles. They contain many small secretory granules, ranging in size from 0.2 to 0.8 micrometers, and IgE receptors are found in the plasma membrane. Mast cells are involved in regulating various physiological functions such as vasodilation, vascular homeostasis, innate and adaptive immune responses, angiogenesis, and venom detoxification. Mast cell activation syndrome (MCAS) is a condition that causes mast cells to release an inappropriate amount of chemicals into the body, causing allergy symptoms and a wide range of other symptoms.