« Understanding Primary Immunodeficiency (PI)

Types of PI

Primary immunodeficiency (PI) diseases are characterized in many different ways, including:

  • Low antibody levels
  • Defects in antibodies
  • Defects in the cells and proteins of the immune system (for example, T cells, B cells, neutrophils, or the complement system)

These defects make people susceptible to recurrent infections, and other complications that may require different therapies. The most common primary immunodeficiency types result in an inability to make a very important type of protein called antibodies or immunoglobulins, which help the body fight off infections from bacteria or viruses. In addition to increased susceptibility to infection, people with PI may also have autoimmune diseases in which the immune system attacks their own cells or tissues.

PI Type:

Severe Combined Immunodeficiency (SCID)


Definition of SCID

Pronounced “skid,” SCID is a rare syndrome with multiple genetic causes, and it is generally considered to be the most serious form of PI. There are currently more than 12 known genetic causes of SCID. These genetic causes differ in respect to the exact mutation causing the SCID, and patterns of inheritance, but they have in common severe deficiencies in T cell and B cell function. For many people, other cells in the immune system are involved as well.

Symptoms of SCID

A high rate of severe infection is the most common symptom in children with SCID. These infections are not the same types of infections, such as colds, experienced by children with normal immune systems. Children with SCID develop infections that are serious and even life threatening, and may include pneumonia, meningitis, or infections of the bloodstream. Widespread use of antibiotics has made it harder for doctors to detect SCID.

Children with SCID are vulnerable to the same infections that affect children with healthy immune systems. In addition, they are at risk of developing infections caused by organisms such as the chicken pox virus (varicella) or vaccines that are not harmful to healthy children. One of the most dangerous infections is Pneumocystis jiroveci, which can cause a rapidly fatal form of pneumonia in SCID patients if not diagnosed and treated promptly.

Children with SCID are also vulnerable to difficult-to-treat fungal infections, such as thrush, a Candida (yeast) infection of the mouth. These infections may improve but not go away, or they may return as soon as medication is stopped.

In addition, children with SCID can develop persistent diarrhea, which can lead to growth failure and malnutrition.

SCID diagnosis SCID diagnosis

Diagnosis of SCID

SCID is usually first suspected in children when they develop serious infections associated with SCID. In some cases, the diagnosis is suspected because there is a previous child in the family with SCID.

Two blood tests are used to reach a diagnosis. The first is a complete blood count. The second test allows the doctor to calculate the total number of lymphocytes in the blood. If the count is still low, additional tests are conducted to count T cells and measure T cell function to confirm the diagnosis.

In some cases, SCID can be diagnosed before a baby is even born. If an analysis has determined which mutation is responsible for a sibling's SCID, genetic testing can be done on cells collected from chorionic villus sampling (CVS) or from amniocentesis. Newborn Screening provides additional information about screening for SCID.

Early diagnosis of SCID, before the infant has had a chance to develop any infections, is very important. If a bone marrow transplant is given during the first three months of life, there is a 94% success rate. Approximately half of the babies born in the U.S. are now being screened for SCID. With routine screening, nearly every infant with SCID could be diagnosed within days of birth. Go to IDF SCID Newborn Screening Campaign to find out if your state is currently screening for SCID.1

Living With SCID

Infants with SCID need to be isolated from children outside the family. If there are siblings who attend daycare, school, or other activities, the possibility of bringing chicken pox into the home is the greatest danger. This has decreased due to widespread use of the chicken pox vaccine, but parents should still let the school know that they need to be notified if chicken pox is in the school. Children who have received the polio vaccine with live virus may spread the virus to a child with SCID, so siblings are advised to receive the killed polio vaccine.

Children with SCID should not receive live vaccines, including rotavirus, chicken pox, mumps, measles, live virus polio, or BCG vaccinations until their immune status has been evaluated. Siblings should not receive the live rotavirus vaccine.

Treating SCID

Only your doctor can determine which treatment is right for you and your specific health needs. Visit our Treating PI section to read about the types of PI treatment and download questions to ask your doctor.