Medical pathologists are usually either clinical or anatomical pathologists. Clinical pathologists mainly examine body fluids, including blood, urine and bone marrow. They may conduct toxicology tests seeking the presence of illegal drugs and poisons, run a hospital blood bank and test patients' immune functions to detect allergies and toleration of transplant organs. Anatomical pathologists analyze samples of tissue and cells to aid in determining the diagnosis and cause of diseases. Once samples have been removed via surgery, biopsies or fine needle aspirations, anatomical pathologists may freeze and examine them under a microscope to view any abnormalities.