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Cancer is a state which involves an abnormal increase in the number of cells, with an potentiality to invade or spread to other parts of the body and destroy their function. Every individual is born with an immune system that acts as a security checkpoint. Immune system do possess B and T lymphocytes,B -cells acts as the military intelligence and T-cells acts as soldiers it also possess some receptors that scans normal cells and abnormal cells like a virus and some other cells. abnormal cells are detected and removed by the T-cells, but in some conditions, T-cells fail to detect and eliminate cancer cells. This is because of higher mutations in cancer cells leads to development of some of the receptors like PD-L1 which acts as the signal jammer of the T-cell.This leads to failure in the work of T-cells against the cancer cells not only this some other receptors like CTLA-4 of T-Cells acts as a type of “off switch” to keep the immune system in off. immune checkpoint inhibitors are used by targeting PD-1, PD-L1 and CTLA-4 targets and drugs like pembrolizumab, nivolumab, ipilimumab and etc. are used to treat various types of cancer.