Chapter 22: Mast Cells and Eosinophils

Introduction

For more than a century physicians have noted a clear connection between mast cell (MC) activation and the subsequent appearance of eosinophils both within the circulation and in tissues. Only recently, however, have basic insights been gained into the mechanisms of this cellular collusion. In keeping with this association, human MCs and eosinophils are considered together in this chapter.

MCs and eosinophils were discovered in the 1870s by the same observer, Paul Ehrlich. He noted that some cells stained in a peculiar fashion when incubated with standard aniline dyes such as toluidine blue and alcian blue. He used the term metachromasie or metachromasia to describe the peculiar color modifications that occurred and the term Mastzellen, meaning “well fed” or “fattened” in German, to describe what we now call MC. Interestingly, this latter term is now known to be a misnomer, since MC cytoplasmic granules are not phagocytosized but rather synthesized during cell growth and again during regranulation. Ehrlich also noted that some cells stained intensely when incubated with the acidic dye eosin. As a result, these cells were called eosinophils. Studies of these two cell types, over the ensuing years, have provided great insight into their roles in biology. They have also highlighted the differences that exist in these cells among different species and their heterogeneity even within a single species and even within single organs.

Mast Cells

The capacity of strategically localized human MC to rapidly release a panoply of powerful chemical mediators makes this cell a unique member of the body's immune response network. Although most frequently discussed in the context of hypersensitivity immune responses, MCs are also known to participate in normal physiological processes including gastric acid secretion,¹ angiogenesis,^2,3 and lipid clearance.^4–8 Increasing evidence supports a role in the innate immune response, especially serving bacterial defense.⁹ MCs also participate in nonallergic pathophysiological processes such as inflammatory bowel disease,^10,11 arthritis,¹² scleroderma,^13,14 tumors, interstitial pulmonary fibrosis,^15–18 envenomation,^3,19–22 and atherosclerosis.^4,5,23,24 Over the years, basophils have been confused with MC in a number of contexts.²⁵ This confusion is due, in part, to a number of similarities between the cells, including the shared expression of FcRI (high-affinity receptor for Fc fragment of IgE), release of preformed histamine, and metachromatic staining. However, MCs are mononuclear cells and are almost exclusively localized to tissues. In contrast, basophils are circulating polymorphonuclear cells that are found occasionally in tissue reactions, including the late-phase allergic response. In addition, significant differences in the two-cell populations exist in cell lineage, ultrastructure, mediator release biochemistry, mediator profiles, pharmacology, and surface antigenicity.

Anatomic Localization

MCs are present in all organs but are particularly abundant in the nose, skin, gastrointestinal tract, and lung. ...