According to the leading experts in immunology, when part of the immune system is either absent or not functioning properly, it can result in an immune deficiency disease. When the cause of this deficiency is hereditary or genetic, it is called a primary immunodeficiency disease (PIDD). Researchers have identified more than 300 different kinds of PIDD.
The immune system is composed of white blood cells. These cells are made in the bone marrow and travel through the bloodstream and lymph nodes. They protect and defend against attacks by "foreign" invaders such as germs, bacteria and fungi.