A T-cell is a type of white blood cell that plays a crucial role in the immune system. It is produced in the bone marrow and matures in the thymus, an organ located behind the sternum. T-cells help the body fight infections and diseases by recognizing and attacking pathogens, such as viruses and bacteria. There are different types of T-cells, including helper T-cells, which assist other immune cells, and cytotoxic T-cells, which directly kill infected or cancerous cells. T-cells are essential for adaptive immunity, allowing the body to remember past infections and respond more effectively in the future.