SCID is a group of very rare and potentially fatal inherited disorders of the immune system. The immune system normally fights off attacks from dangerous bacteria and viruses. People with SCID have a defect in their immune system that leaves them vulnerable to potentially deadly infections.
There are several types of SCID. The most common form is caused by a mutation in the SCIDX1 gene located on the X chromosome. This gene encodes a protein that is used to build a receptor called IL2RG (interleukin-2 receptor). These receptors sit in the plasma membrane of immune cells. The receptors job is to allow two types of immune cellsT cells and B cellsto communicate. When the gene is mutated, the receptors cannot form and are absent from immune cells. As a result, the immune cells can’t communicate with one another about invaders. Too few T and B cells are made to fight off the infection, and the body is left defenseless. Another form of SCID is caused by a mutation on chromosome 20 and is characterized by a deficiency of the enzyme adenosine deaminase.