IL9R and infection: To address the possibility that constitutive IL-9R deficiency would lead to infection-independent intrinsic defects in the mast cell compartment, we compared the degranulation response, quantified as CD107a translocation from secretory granules to the plasma membrane, of WT and IL-9R−/− bone marrow-derived mast cells (BMMC) to different stimuli in vitro (Fig. 3).