Eosinophils are mainly tissue cells, attracted at mucosal sites by several chemotactic factors. They express numerous surface receptors, which make them sensitive to several differentiation, activation and inflammatory signals. Eosinophilic specific granules contain highly cationic proteins, which are released upon activation by immune effectors, like immune complexes or by inflammatory mediators. These basic granule proteins are cytotoxic for non-self targets such as parasite larvae but also for normal cells or tissues, giving to eosinophils a dual function in immune defense and in immunopathology. Eosinophils are also a source of various mediators including growth factors, proinflammatory cytokines, chemokines, and immunoregulatory molecules, conferring to eosinophils multiple functions, not only as effector and pro-inflammatory cells but also as immunoregulatory cells.