CCL22 and allergic asthma: The mechanism could explain why Th2 cells migrate to asthma airways as T cells in bronchial mucosal or bronchoalveolar lavage fluid (BALF) of allergic asthma express CCR4, and meanwhile the levels of CCL22 in BALF but not Th1-selective chemokines are increased upon allergen challenge to the lung [34–37].