What is a key characteristic of T cells in the immune system?

A key characteristic of T cells in the immune system is their ability to directly kill virus-infected cells. This function is primarily carried out by a subset of T cells known as cytotoxic T lymphocytes (CTLs), or CD8+ T cells. When a CTL recognizes a cell that is presenting a foreign antigen on its surface (often from a virus), it binds to that cell and induces apoptosis, or programmed cell death. This process helps to eliminate infected cells and prevent the spread of the virus, highlighting the crucial role T cells play in the adaptive immune response.

Other functions of T cells include helping to regulate immune responses through helper T cells (CD4+ T cells), which secrete cytokines to assist other immune cells, but the direct killing of infected cells is a distinctive and vital characteristic of cytotoxic T cells. This aspect of T cell function is essential for controlling viral infections and shaping the overall immune response.