Anti-BTLA treatment can promote T-cell proliferation, and BTLA knockout mice show higher immune activity.199–201 In patients with malignant melanoma, tumor-specific T cells in both circulating lymph and metastatic lymph nodes expressed BTLA, and the expression of HVEM was also detected in the patient’s tumor tissue.202 The expression of BTLA was found to be significantly increased in pleural effusion samples from patients with lung cancer, which is an indicator of tumor aggressiveness.203 Therefore, BTLA, as an inhibitory molecule for immune regulation, has broad research prospects. Here, BTLA is linked to Pleural effusion.