White blood cells are produced within the bones in a substance called bone marrow, according to the Health Encyclopedia of the University of Rochester Medical Center. White blood cells only account for around 1 percent of a human's blood.
White blood cells help a person fight off bacteria and viruses, and they are essential for a strong immune system. Once produced within the bone marrow, they are stored in the lymphatic system of the body, and due to their function, each cell only lasts from 1 to 3 days. This means that the bones must constantly produce new white blood cells to replenish the supply.
There are five different types of white blood cells: monocytes, lymphocytes, neutrophils, eosinophils and basophils. Monocytes help break down bacteria, and lymphocytes defend against viruses and bacteria. Neutrophils kill fungi and bacteria, and they are the first ones to "battle" foreign invaders. Eosinophils attack cancer cells, aid in allergic responses and kill parasites, and basophils send out histamine when needed.