For histologic examination, intradermal challenges on the abdomen skin were inducted on Day 40. After 20 min, 24 h and 48 h, the mice were sacrificed using cervical dislocation, and the challenge site was removed and placed in 10% neutral buffered formalin overnight at room temperature. The skin tissue was dehydrated gradually through 70%, 80% and 95% and absolute ethanol (Guo et al., 2020 (link)), then soaked in xylene for 1 h at 55–60°C followed by embedding in paraffin for 1 h. The specimens were sectioned to a thickness of 5 µm using a Leica RM2235 microtome (Leica) and followed by deparaffinisation and haematoxylin and eosin stain. The sections were also stained with rabbit anti‐CD4 (1:200) and rat anti‐CD8 (1:200) monoclonal antibodies (NovusBiologicals) to detect cluster of differentiation (CD)‐maker positive cells: The sections were incubated with peroxidase‐labelled goat anti‐rabbit and goat anti‐rat antibodies and stained in the substrate solution containing 3,3′‐diaminobenzidine (ZSGB‐BIO). Inflammatory cell infiltration was examined using light microscopy, and the images were taken using the microscopic camera system BA400 Digital (Motic Group Co. Ltd.).