This book looks in detail at human mast cells. It includes a review of previous and current ultrastructural studies, whereby the latter are illustrated with numerous high-quality electron micrographs obtained from a large number of structural and functional experiments using highly purified isolated human lung mast cells. Specifically, the book provides criteria for the identification of human mast cells, discusses the unique role in mast cell function of granules and lipid bodies, describes the ultrastructural anatomy of two release reactions - i.e., anaphylactic degranulation and piecemeal degranulation - and defines the recovery, cyclical and maturational properties of human mast cells. All of these new morphological-biochemical and functional studies are correlated with the author's wide experience in the visual properties of human mast cells as seen in biopsy material obtained from a diagnostic ultrastructural pathology service.