- A type of white blood cell that is found in connective tissues all through the body, especially under the skin, near blood vessels and lymph vessels, in nerves, and in the lungs and intestines. Mast cells play an important role in how the immune system responds to certain bacteria and parasites and they help control other types of immune responses. They contain chemicals such as histamine, heparin, cytokines, and growth factors. They release these chemicals during allergic reactions and certain immune responses. These chemicals have many effects, including the widening of blood vessels and angiogenesis. During an allergic response, they can cause flushing (a hot, red face) and itching. In large amounts, they can also cause abdominal cramps, muscle pain, nausea, vomiting, diarrhea, low blood pressure, and shock. Mast cells are part of the body’s immune system.