Cytotoxic T-lymphocyte: A T cell that is antigen-specific and is able to search out and kill specific types of virus-infected cells. When cytotoxic T-lymphocytes (CTLs) find cells carrying the viral peptide they are looking for, they induce these cells to secrete proteins that attract nearby macrophages (a type of white blood cells). These macrophages then surround and destroy the infected cells. CTLs are important in the body's response to viruses and cancer.

CTLs expresses the CD8 transmembrane glycoprotein and are therefore also known as CD8+ T cells or CT cells.