Hematopoietic stem cells.

The hematopoietic system of the young child acquires, through time, the ability to cope with exposure to a number of environmental toxins and infectious agents. Occasionally, severe aplastic anemia occurs secondary to exposure to some of these toxins or infectious agents. The occurrence of severe aplastic anemia provides an opportunity to study the maturation of the hematopoietic system because often the immune system is partially intact. Hematopoietic stem cell transplants permit the study of the complete reconstitution of the hematopoietic and immunologic system. Stem cell transplants are often used to treat severe aplastic anemia or, alternatively, may be part of the treatment for an underlying malignant disease or a genetic disease. Sources of stem cells and the age of the recipient and donor have an impact on the success of the stem cell transplant. A stem cell transplantation provides a window of opportunity to study and observe the normal maturation of the immune system and the sensitivity. Very clearly, children recover from severe aplastic anemia and stem cell transplantations more readily with fewer problems and complications than adults. The environmental risks that a child who received a stem cell transplantation faces are related primarily to the deficiencies of the hematopoietic system and immune system during the recovery phase. Therefore, diminished resistance to infectious agents, primarily viruses and other opportunistic organisms, are the primary risk that children who are recovering from these transplantations face. There are few data on the susceptibility of these children to the toxic effects of other environmental toxicants during the recovery period, which may take years before complete recovery.