A stem cell is a cell with the unique ability to develop into specialised cell types in the body. In the future they may be used to replace cells and tissues that have been damaged or lost due to disease.Stem cells are the foundation from which all parts of the human body grow. Stem cells are important for living organisms for many reasons. In the 3- to 5-day-old embryo, called a blastocyst. An immature cell that can develop into all types of blood cells, including white blood cells, red blood cells, and platelets.There are several types of stem cells that can be used for different purposes. Embryonic stem cells. Embryonic stem cells come from human embryos that are three to five days old. Non-embryonic (adult) stem cells. Induced pluripotent stem cells (iPSCs) Cord blood stem cells and amniotic fluid stem cells. Stem cells are special human cells that are able to develop into many different cell types. This can range from muscle cells to brain cells. In some cases, they can also fix damaged tissues. Researchers believe that stem cell-based therapies may one day be used to treat serious illnesses such as paralysis and Alzheimer disease. The only stem cells now used to treat disease are hematopoietic stem cells. These are the blood cell-forming adult stem cells found in bone marrow. Every type of blood cell in the bone marrow starts as a stem cell. Stem cells are immature cells that are able to make other blood cells that mature and function as needed.