Eosinophils are key effector inflammatory cells in allergic asthma that are characteristically recruited to the lung in the setting of adaptive immune responses mediated by Th2-type CD4+ T cells, whereas neutrophilic airway inflammation has been associated with corticosteroid treatment in severe asthma and can be mediated by Th1- or Th17-type T cells (see Box 2) (Barnes, 2008; Holgate, 2012; Kim et al., 2010; Wenzel, 2012a). This evidence concerns the gene CD4 and asthma.