Severe Combined Immunodeficiency (SCID)

Nevada Newborn Screening Program

Newborn Screening for Severe Combined Immunodeficiency

Severe Combined Immunodeficiency (SCID) is the name for a group of inherited disorders that cause babies to be born without a working immune system. These disorders are also called primary immune deficiency disorders (PID). SCID is also sometimes called:

  • Severe combined immune deficiency
  • Severe mixed immunodeficiency syndrome
  • Primary immune deficiency
  • Bubble boy disease

Newborns with SCID may seem healthy at first because their mother's immune system protects them from infections for the first few weeks of life. However, without necessary treatment, common infections and vaccines can be life threatening to these infants. The immune system functions with the help of lymphocytes. Lymphocytes are a type of white blood cell made by the bone marrow. There are two types of lymphocytes: T cells and B cells. People with SCID have a severe defect in their T cells, and B cells that do not work well. People with SCID are not able to fight off common infections. They also cannot make antibodies to protect themselves.

Frequently Asked Questions

What are the Symptoms of SCID?

Symptoms of SCID are usually seen in the first year of life and may be fatal. It is important to remember that each child with SCID is different and may experience symptoms differently. Symptoms of SCID may include:

  • Failure to thrive (not gaining weight, or not growing at a healthy rate)
  • Serious and/or life-threatening infections that happen more than once, are not easily treated, and do not get better with medicine. These infections can include: meningitis (brain infection), pneumonia (lung infection), sepsis (blood infection)
  • Opportunistic infections (infections that do not cause disease in healthy people). These can include: pneumocystis pneumonia (a specific type of pneumonia), fungal infections, viral infections
  • Other infections that may happen more frequently include: otitis media (ear infection),sinusitis (sinus infection), skin rashes/infections
How Does Someone Get SCID?

Genes tell the body how to function. The body normally makes T cells and B cells. People with SCID can have a change in one or more of at least 21 different genes. The gene changes cause the body to have little or no T cells and B cells. This means that the body is not able to defend itself and make the antibodies needed to fight infections. This causes common infections, like a cold or flu, to make the child very sick or may even lead to death.

We cannot get them by eating. SCID is not contagious. You cannot get SCID from living with, touching, or spending time with someone who has SCID.

Who is Affected by SCID?

The true incidence of SCID is unknown. It is estimated that there is one new case per 50,000 to 100,000 live births. Infants with SCID are highly susceptible to infections and may not be diagnosed until these infections prompt the pediatrician to examine their immune system, usually after 4 months of age.

It seems that one of the autosomal recessive forms of SCID happens slightly more often in people of Native American ethnicity, but it can affect people of all ethnic backgrounds.

How is SCID Inherited?

SCID is inherited in different ways depending on the type of SCID and which gene is causing it. The most common way SCID is inherited is called X-linked recessive. SCID can also be inherited in an autosomal recessive manner. When both parents are carriers, there is a 25% chance in each pregnancy for the child to have SCID. There is a 50% chance for the child to be a carrier, just like the parents. There is a 25% chance for the child to have two working genes. Brothers and sisters who do not have SCID still have a chance of being carriers, like their parent(s). Except in special cases, carrier testing should only be done on people over 18 years of age.

Depending on the type of SCID, each healthy brother and sister has a 50-66% chance of being a carrier for SCID. In some cases, siblings should also be evaluated for SCID. It is important for other family members to be told that they could be carriers. There is a chance they are also at risk to have children with SCID.

In some families, SCID is not inherited. This means that the baby is the first in the family to have SCID.

Confirmatory Testing

There are two types of testing used to confirm SCID. These tests are called a ‘complete blood count' (CBC) and ‘flow cytometry'. These need a blood sample to measure the amount of lymphocytes in an infant's blood.

Infants who are confirmed to have SCID are referred to a pediatric immunologist or infectious disease specialist. The specialist will make recommendations for further treatment and management.

What is the Treatment for SCID?

For infants with SCID, the focus is to prevent infections, treat any active infections, and avoid live vaccines.

For prevention, antibiotics and immune globulin may be used. Immune globulin (also called immunoglobulin or gamma globulin) gives the body antibodies that are normally made by the B cells. These antibodies help fight infections.

For active infections, aggressive antibiotic, antifungal, and antiviral medicines may be used.

Medicines and immune globulin are only short-term treatment options. They can only keep patients with SCID healthy temporarily.

Patients with SCID need treatments that are more permanent. These include:

  1. Bone marrow transplantation (BMT): In most cases of SCID, children will need a bone marrow transplant (BMT). This is the only treatment that gives children a working immune system that is able to prevent and fight infections.
  2. Enzyme replacement therapy (ERT): Some cases of SCID are caused by a shortage of an enzyme called adenosine deaminase (ADA). ADA is found throughout the body, but is most active in lymphocytes. A shortage of this enzyme leads to low numbers of T cells and B cells. Enzyme replacement therapy (ERT) with pegademase bovine (PEG-ADA) allows lymphocytes to recover. It may be a long-term treatment option for some children. It is not considered a cure, however, and BMT may still be needed.
  3. Gene therapy: Some clinical trials have shown positive outcomes for children, but this form of therapy is still being studied. Gene therapy for the type of SCID with low ADA has produced very good results. For other forms of SCID, gene therapy is still an experimental treatment. It is available at a few hospitals in the USA.