Immunohistochemistry is an immunochemical test used to detect the expression of the BCL2 (B-cell lymphoma 2) protein in cells. BCL2 acts as an inhibitor of apoptosis (programmed cell death), and its increased expression prevents cell death and allows abnormal cell survival. This test is primarily used in the diagnosis and classification of lymphomas (especially follicular lymphoma and its variants), and in the study of other tumors such as breast and prostate cancer.