CD4 and head and neck squamous cell carcinoma: All immune cells except mucosal-associated invariant T were significantly associated with HLA-DPA1 and CD4 expression in HNSCC patients (Table 5) (all P < 0.05), and the ssGSEA algorithms also revealed that all 23 immune cells infiltration abundance had significant associations with both the expression of HLA-DPA1 and CD4 (Additional file 3: Figure S3C) (all P < 0.05).