Mast cells were initially believed to have a nutritive function, but rapidly have been found to play key roles in the defense against helminths and in immediate type hypersensitivity reactions. Unfortunately, research on mast cells is limited by the fact that they cannot be found in the peripheral blood, so that basophils are sometimes used instead, mainly because they share with mast cells the high affinity IgE receptor. After a short overview on historical research results and well known concepts of IgE mediated hypersensitivity reactions, this thesis focusses on novel insights of mast cells disease. Particularly, the use of laboratory techniques to identify conditions such as immediate type hypersensitivity reactions, mastocytosis, mast cell activation syndromes, hereditary alpha tryptasemia and pseudo-allergic reactions will be discussed in order to introduce already published research results and ongoing projects.