Illustration of three types of white blood cell responding to infection by bacteria (green): macrophages (purple), dendritic cells (blue) and t helper cells (red). The T helper cells have released signalling proteins called cytokines (numerous red spheres) which attract other immune cells and coordinate the inflammatory response. Macrophages mainly function to engulf and destroy substances which provoke an immune response. This process is known as phagocytosis. Dendritic cells primary function is presenting antigens, which are molecules which the immune system recognises as foreign.