Hematoxylin–eosin staining
After mice were sacrificed, their entire heads and spleens were soaked in 10% neutral formalin fixative for 24 h, because the nasal cavity of BALB/c mice is small. Afterwards, the heads were soaked in 0.5 M ethylene diamine tetraacetic acid for decalcification for 4 weeks. After the mouse skull was softened, the tissues were conventionally dehydrated, embedded in paraffin, and sectioned in the coronal position (about 3 μm thick slices). The histopathological changes of the nasal mucosa were visualized with hematoxylin–eosin staining. The same steps were applied to the spleens.