Illustration showing white blood cells responding to bacteria (green) entering the body through a breach in the skins outer layer (epidermis, pink) caused by a needle (top centre). Two types of white blood cell are present: dendritic cells (blue) and T helper cells (red). White blood cells recognise substances that should not be present in a healthy human body and help to nullify them. These cells are releasing cytokines (orange dots), small signalling proteins, in response to the bacteria. Cytokines are important in attracting more white blood cells once an infection has been found and coordinating the immune response.