T cells are a type of white blood cell that are a crucial part of the adaptive immune system, responsible for fighting infection, cancer, and autoimmune diseases. They originate in the bone marrow, mature in the thymus, and are trained to recognize foreign invaders while ignoring the body’s own cells. The two main types are cytotoxic T cells, which directly kill infected or cancerous cells, and helper T cells, which coordinate the immune response by signaling other immune cells. Functions of T c