A mast cell are type of white blood cell, also known as a mastocyte or a labrocyte. Mast cell are granulocyte derived from the myeloid stem cell that is a part of the immune and neuroimmune systems and contains many granules rich in histamine and heparin. A mast cell are type of white blood cell, also known as a mastocyte or a labrocyte. Mast cell are granulocyte derived from the myeloid stem cell that is a part of the immune and neuroimmune systems and contains many granules rich in histamine and heparin.