IL4 and allergic disease: However, in the context of allergy, it appears that C57BL/6 mice are more prone to generating Th2 responses in the lung while BALB/c mice generate Th1 responses, despite high levels of IL-4 and IL-5 produced by BALB/c splenocytes.180 The impact of strain genetic background on local and systemic immunity during breakthrough infection should be investigated further.