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.

Allergies and anaphylactic shock arise when mast cells are not limited to the local site of injury and the immunoglobulin activator cascades either to a larger area or throughout the body.

Mast cell at Wikipedia

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