What are mast cells?

MastCellAware.com has this good answer to the question, “What are mast cells?”

I found their website when I was trying to learn about mast cell degranulation. As they state, “Each mast cell contains secretory granules (storage sacs), each containing powerful biologically active molecules called mediators. These can be secreted when mast cells are triggered, leading to allergic and inflammatory diseases.”