An immunologist is a doctor who is an expert on the immune system. Any pathogen or other intruder is met with a response by your body. This response mobilizes many systems including the cardiovascular, nervous, and digestive systems. Special cells called lymphocytes are mobilized to deal with any incoming threat. Furthermore, in the case of an injury, platelets are used to coagulate the blood in order to stop any bleeding. Immunology focuses on how people can maximize the effectiveness of their immune systems to fight off infection.