1 Garry M. Walsh (ed.), Eosinophils: Methods and Protocols, Methods in Molecular Biology, vol. 1178, DOI 10.1007/978-1-4939-1016-8_1, © Springer Science+Business Media New York 2014 Chapter 1 Eosinophil Overview: Structure, Biological Properties, and Key Functions Paige Lacy, Helene F. Rosenberg, and Garry M. Walsh Abstract The eosinophil is an enigmatic cell with a continuing ability to fascinate. A considerable history of research endeavor on eosinophil biology stretches from the present time back to the nineteenth century. Perhaps one of the most fascinating aspects of the eosinophil is how accumulating knowledge has changed the perception of its function from passive bystander, modulator of inflammation, to potent effector cell loaded with histotoxic substances through to more recent recognition that it can act as both a positive and negative regulator of complex events in both innate and adaptive immunity. This book consists of 26 chapters written by experts in the field of eosinophil biology that provide comprehensive and clearly written protocols for techniques designed to underpin research into the function of the eosinophil in health and disease. Key words Eosinophil, Accumulation, Apoptosis, Degranulation, Animal models 1 Introduction Eosinophil involvement in inflammatory conditions affecting the skin, gastrointestinal tract, and upper and lower airways is well documented [1]. Although asthma is recognized as a heteroge- neous condition, eosinophilic asthma is a recently described phe- notype of the disease characterized by increased blood or sputum eosinophils [2] whose numbers correlate with disease severity [3]. Infiltrating tissue eosinophils release their potent pro-inflammatory arsenal including granule-derived basic proteins, lipid mediators, cytokines, and chemokines. These contribute to airway inflamma- tion and lung tissue remodelling that includes epithelial cell damage and loss, airway thickening, fibrosis, and angiogenesis [4]. More recent evidence suggests that in addition to their role as degranu- lating effector cells, eosinophils have the capacity to act as antigen- presenting cells resulting in T cell proliferation and activation thereby propagating inflammatory responses [5, 6].