Eosinophilic and noneosinophilic asthma

Asthma can be subdivided into eosinophilic or non-eosinophilic phenotypes based on the inflammatory cellular patterns seen in sputum, blood, and airway tissue compartments. However, this level of stratification does not always produce robust clinical phenotypes as there is considerable overlap across these phenotypes when viewed from the perspective of ``eosinophilia''. Multiple proposed subphenotypes of asthma have instead been described within both the eosinophilic and non-eosinophilic groups. An endotype is the mechanism driving a subphenotype including those associated with eosinophilia such as early-onset allergic asthma, with and without obesity, aspirin sensitive asthma and late-onset eosinophilic asthma, allergic bronchopulmonary mycosis, and exercise-induced asthma. Conversely, subphenotypes of asthma not associated with eosinophilia include those of obesity-induced asthma, asthma with fixed airflow obstruction and minimal inflammation (paucigranulocytic), neutrophilic asthma, and asthma associated with smoking, pollution, infection, or other respiratory irritants. Each of these subphenotypes may have one or more endotype or mechanism driving their clinical presentation. We review the known mechanisms that define each endotype and that lead to the clinical presentation of the known eosinophilic and non-eosinophilic subphenotypes. This approach yields important information regarding potential preventative or therapeutic interventions in these patients with asthma.