Animation of antigen-presenting cells (APCs, blue) delivering antigens from invading organisms to a T-cell (grey). APCs are immune system cells that ingest pathogens in the body, and isolate unique identifying proteins from them. The APCs then bind the protein fragments to a major histocompatibility complex (MHC) protein on the cell surface, and presents the complex to a T-cell, activating it. The activated T-cells then divide and produce chemicals (cytokines) that direct the immune response to the pathogen. This process is part of the adaptive immune system, and is critical in preventing infection. It is also the route by which vaccination works.