BTLA and cancer: Programmed cell death protein 1 (PD‐1), cytotoxic T‐lymphocyte‐associated protein 4 (CTLA‐4), lymphocyte‐activation gene 3 (LAG3), T‐cell immunoglobulin mucin‐3 (TIM‐3), T‐cell immunoglobulin and ITIM domain (TIGIT), and B and T lymphocyte attenuator (BTLA) are just a few of the inhibitory immunoreceptors that have already been discovered and researched in cancer throughout the recent years.