Mast cells are cells found throughout the body that contain histamines and heparin. They are a key part of the body's immune system, but also play a role in many autoimmune diseases, particularly allergies and mastocytosis. Although they are structurally and functionally similar to a type of white blood cell, they arise from different cell lines. Mast cells can be activated in a few different ways, such as direct injury, immunoglobulin levels or activator proteins.