Severe combined immunodeficiency, also known as SCID, is a genetically inherited disorder of the immune system. Children born with the disease have a severely deficient immune system. Exposure to microorganisms, even those that are not usually pathogenic in humans, can cause serious infections in people with severe combined immunodeficiency syndrome.
Around 50% of people with SCID have X-linked severe combined immunodeficiency. This form of the disorder is caused by a mutation in an X-chromosome gene which codes for a protein that is part of a receptor for several cytokines which are essential for immune function. The remaining 50% have other variations of SCID which are caused by mutations in autosomal genes.
Regardless of the genetic cause, people with severe combined immunodeficiency disorder have immune systems with T cells and B cells which are severely dysfunctional. Due to these dysfunctions, their immune systems are incapable of developing memory responses. Depending on the type of SCID involved, other types of immune cells, such as natural killer cells, may also be non-functional.
Children who are born with SCID die of severe infections within two years if they are not diagnosed and treated. Most children are diagnosed within six months of birth, usually as a result of recurrent severe infections. Other symptoms of the disease include poor feeding and low weight gain after birth, dehydration, and chronic diarrhea. Diagnosis tends to be delayed because for the first few months of life, babies with SCID still carry antibodies acquired from their mothers before birth.
When a child is diagnosed with severe combined immunodeficiency disease, he or she is immediately treated with intravenous immunoglobulin. This is a liquid solution of protective antibodies that will help prevent infection. In some cases prophylactic antibiotics and other medications may also be administered. For example, most children diagnosed with SCID are given antibiotic treatment to prevent infection with Pneumocystis jiroveci, which can cause pneumonia in infants.
The second step that is carried out upon diagnosis of SCID is the child being evaluated as a candidate for bone marrow transplant. In this procedure, a donor is found who matches the child’s tissue type. Donor stem cells are transplanted into the child, where it is hoped that the cells will form a stable population of immune cell precursors. If the transplant is successful the child’s immune system is restored, and his or her bone marrow will continue to produce functional T cells and B cells. In general, the younger a child is when the transplant is carried out, the greater the chance that the procedure will be successful. The optimum age for transplant is three and a half months or younger.