They are most easily resolved histologically after phagocytizing substances such as ink particles or ferritin. They have abundant lysosomes , reflecting their active role in digesting scavenged material. When stimulated they can also be active in secretion and then have well developed RER and Golgi. They, too, originate in bone marrow and circulate to connective tissue, but reside there for weeks to months. When surface receptors for IgE antibodies bind cognate antigen, granule release is triggered, initiating immediate hypersensitivity reaction.
Histamine causes vasodilation and increased vascular permeability.
Mast cells are typically located near blood vessels , thus vessel response to released substances is rapid. Mast cells are round to ovoid, 20-30 microns in diameter. These include all blood leukocytes , some of which reside in connective tissue for their entire functional life span. For example B lymphocytes differentiate into tissue plasma cells, monocytes differentiate into tissue macrophages, and eosinophils remain with their characteristic appearance. T and B types are indistinguishable without special staining techniques.