Severe Combined Immunodeficiency (SCID)

Severe combined immunodeficiency (SCID) is the most serious form of primary immunodeficiency (PID). Most infants with SCID are diagnosed within the first year of life, and require a haematopoietic stem cell transplant (HSCT) to survive. Early diagnosis by newborn screening for SCID allows for a HSCT transplant to be undertaken before infections cause complications, that may be life threatening.

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SCID is usually an inherited disorder

SCID and other PIDs are caused by defects in cells of the immune system, and are usually inherited. PIDs are different to AIDs (acquired immunodeficiency syndrome), that is due to human immunodeficiency virus (HIV).

Children usually inherit SCID from their parents, by either of the following ways:

What is SCID?

The main role of the immune system is to defend against infections and other foreign invaders (such as cancer cells), whilst protecting the body's own cells. Around half the white blood cells in healthy people are T cells (T lymphocytes), which are the most important cells in the immune system.

One of the roles of T cells is to help another type of white blood cell called B cells (B lymphocytes), to produce antibodies, also known as immunogobulins. In SCID neither the T cells nor the B cells work properly.

Blood from infants with SCID does not usually have any T cells. Even if the blood of infants with SCID contains B cells, the B cells cannot make antibodies without T cells.

Infants are usually protected at birth against infections like tetanus, diphtheria, chickenpox, polio and most types of meningitis. After birth, antibodies start to gradually disappear from the infant's blood, and by 6 months of age they are practically gone. The amount of antibodies in the blood is shown by the Immunoglobulin G (IgG) level.

Infants with SCID can't produce IgG, so once the IgG from the mother has gone, they easily get infections that antibodies prevent.

Infants with SCID tend to at risk of severe infections of the lungs, especially by Pneumocystis carinii or by cytomegalovirus (CMV). Symptoms include poor growth rate and chronic (ongoing) diarrhoea.

How is SCID diagnosed?

The diagnosis of SCID in infants is based on several signs, including:

Newborn screening for SCID is routinely performed in New Zealand, the United States and in some European countries. Early diagnosis by newborn screening for SCID allows for a HSCT transplant to be undertaken before infections cause complications, that may be life threatening.

Treatment options depend on the cause of SCID

Depending on the cause of SCID, there are three main treatment options, in addition to specific treatment of infections with appropriate antibiotics:

Deficiency of adenosine deaminase enzyme (ADA SCID)

Missing antibodies or immunoglobulins

T cell disorders

Patient support organisations

The following organisations provide support for people with SCID and their families:

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