T-cells are a type of white blood cell that play a crucial role in the immune system. They originate from stem cells in the bone marrow and mature in the thymus, an organ located behind the sternum. T-cells are essential for identifying and attacking pathogens, such as viruses and bacteria, as well as cancer cells. They can be classified into different types, including helper T-cells, which assist other immune cells, and cytotoxic T-cells, which directly kill infected or cancerous cells. The activation of T-cells occurs when they recognize specific antigens presented by antigen-presenting cells. This recognition triggers a series of responses, leading to the proliferation and differentiation of T-cells. Once activated, T-cells can form memory cells, which provide long-lasting immunity by remembering past infections. This ability to remember and respond quickly to previously encountered pathogens is a key feature of the adaptive immune system.