Severe combined immunodeficiency (SCID) is a group of inherited immune system disorders in which some of the cells that fight infections are missing or do not work well. The immune system is made up of special cells, tissues, and organs. It helps the body fight infections. In these disorders, some of the cells in the immune system that fight infections (T and B cells) are missing or do not work well. The disorders are inherited and are there at birth.
Infants born with SCID do not gain weight or grow at a healthy rate, which doctors call failure to thrive. An infant with SCID will have a lot of severe bacterial, viral, or fungal infections that often keep coming back after they are treated with drugs. If left untreated, babies with severe SCID will usually die from infections within a year.
These disorders are rare. In the United States, they affect about one out of 100,000 babies born. There are about 100 different types of SCID.
Types of SCID
Two of the most common types of SCID are classical X-linked SCID and adenosine deaminase (ADA) deficiency SCID.
Classical X-linked SCID – A large number of patients with SCID have classical X-linked SCID (sometimes called “bubble boy” disease). Patients with this type of SCID have very few working white blood cells to fight infection. Only boys can have this type of SCID.
ADA deficiency SCID – A small number of patients with SCID have ADA SCID. These patients have low levels of an infection-fighting molecule in the body called ADA. Boys and girls can have this type of SCID. A few forms of ADA SCID don’t cause problems until later in childhood, adolescence, or even adulthood.
1. Chan K, Puck J.M. Development of population-based newborn screening for severe combined immunodeficiency. J Allergy Clin Immunol. 2005; 115(2): 391-398.