Disease pathogens. Computer illustration of two types of pathogen (yellow spiky and purple) in the bloodstream, with a white blood cell (leucocyte, light blue, upper right) of the immune system. Pathogens are invading micro-organisms which can cause disease. The leucocyte cell forms part of the body's immune response, used to keep the blood free from pathogens and abnormal body cells. Also present are the oxygen-carrying red blood cells (erythrocytes, red) and the platelets (yellow), involved in blood clotting.