A T-cell is a type of white blood cell that plays a crucial role in the immune system. It is responsible for identifying and attacking infected or cancerous cells in the body. T-cells originate from stem cells in the bone marrow and mature in the thymus, which is where they gain their name. There are different types of T-cells, including helper T-cells and cytotoxic T-cells. Helper T-cells assist other immune cells by releasing signaling molecules, while cytotoxic T-cells directly kill infected or abnormal cells. Together, they help maintain the body's defense against diseases and infections.