Homing and migration
MC progenitors circulating in the blood, in humans defined as CD34+, KIT+, FcεRI+ cells 68, enter a specific tissue where they complete their maturation under the influence of locally produced factors. Surprisingly, the homing mechanisms for MC progenitors are still mostly unknown, except that it is a tissue specific process 69-71. In addition to their basal homing, MC numbers clearly increase at sites of inflammation, such as in allergic rhinitis and asthma. This increase in MC numbers is likely a consequence of both increased proliferation and survival, as well as maturation of their progenitors recruited in response to chemoattractants 72,73. The fact that some chemokine receptors, such as CCR1, CXCR2 and CXCR4, are expressed on both progenitors and mature MCs suggests that MC accumulation at sites of inflammation may also involve relocalization of mature MCs within a specific tissue 71-76.