Eosinophils used to be thought of as regulators of allergic inflammation, but there is now evidence to the contrary; eosinophils have been found to be the major effector cells responsible for the late asthmatic response (LAR), airway hyperresponsiveness (AHR) and, at least in part, airway remodeling by releasing leukotrienes and highly basic and cytotoxic proteins such as major basic protein (MBP). However, a recent clinical trial using humanized anti-interleukin-5 monoclonal antibody found a failure to reduce AHR and the LAR, whereas the antibody entirely abolished tissue eosinophilia. In addition, abundant MBP has recently been found in mast cells as well as in eosinophils by our transcriptome (the whole transcripts that a cell expresses) screening of all leukocyte types. Eosinophils are indeed unlikely to be involved in the LAR and may not be unique cytotoxic cells in asthma. It is now necessary to determine the real role of eosinophils and whether early intervention to block eosinophil recruitment into the asthmatic lung does prevent airway remodeling and AHR.

Matsumoto, K., & Saito, H. (2001). The Role of Eosinophils in Asthma: Sarastro or the Queen of the Night? International Archives of Allergy and Immunology, 125(4), 290–296. Portico.