CID, or combined immunodeficiency, is an inherited immune disorder in which both T cells and B cells, the two main branches of the adaptive immune system, fail to work properly. This leaves the body unable to fight off infections effectively. CID belongs to a larger family of more than 400 genetic immune disorders collectively called inborn errors of immunity, and it sits on a spectrum that ranges from moderate immune dysfunction to the life-threatening form known as severe combined immunodeficiency (SCID).
If you searched “CID disease,” you may also have encountered references to CIDP, or chronic inflammatory demyelinating polyneuropathy, a completely different condition that affects the peripheral nerves rather than the immune system. This article covers combined immunodeficiency.
How CID Differs From SCID
The word “combined” in CID means that T cells and B cells are both affected. T cells coordinate the immune response and directly kill infected cells, while B cells produce antibodies. In SCID, T cells are nearly absent, with counts below 0.05 × 10⁹ per liter of blood. In CID, T cells are usually detectable but dysfunctional. Some patients with CID characteristically lack one subset of T cells (CD8 cells) while retaining another (CD4 cells), though the ones present don’t work correctly.
This distinction matters for newborn screening. In the United States, newborn blood spot tests look for a marker called TRECs, which reflects T-cell production. Because SCID babies have almost no T cells, the screening catches them reliably. CID, however, often slips through because T cells are present, just impaired. Patients with CID who have profound T-cell dysfunction frequently have normal TREC levels and escape detection at birth.
The clinical severity of CID is highly variable. Some children experience disseminated, life-threatening infections similar to SCID, while others have milder infections that respond to standard treatment and never require hospitalization.
What Causes CID
CID is genetic. Mutations in specific genes disrupt the development or signaling of T cells, and because T cells help B cells function, antibody production suffers as well. More than 430 genetic defects have been linked to inborn errors of immunity overall, and many of those fall within the combined immunodeficiency category.
Some well-characterized mutations include defects in the CD3 complex, a group of proteins that sits on the surface of T cells and transmits signals when the cell recognizes a threat. Mutations in the CD3-delta, CD3-epsilon, or CD3-zeta genes each impair T-cell development and signaling in slightly different ways. ZAP70 deficiency, another recognized cause, disrupts a protein essential for T-cell activation. MHC class II deficiency prevents immune cells from presenting threats to T cells in the first place, effectively blinding part of the immune system.
Most forms of CID follow an autosomal recessive inheritance pattern, meaning a child must inherit a defective copy of the gene from both parents to develop the disease.
Syndromic Forms of CID
Some genetic mutations affect not only immune cells but also other organ systems, producing combined immunodeficiency alongside other recognizable features. These are classified separately as “combined immunodeficiencies with associated characteristics or syndromes.”
DiGeorge syndrome is one of the most recognized examples. It results from a chromosomal deletion that affects the thymus (where T cells mature), the heart, and facial structures. CHARGE syndrome, Bloom syndrome, ataxia-telangiectasia, and Ligase IV syndrome are other conditions in this category. In each case, the immune deficiency is just one piece of a broader clinical picture that can include developmental differences, neurological problems, or organ malformations.
Common Symptoms
CID typically becomes apparent in infancy or early childhood, though milder forms may not be recognized until later. The hallmark is recurrent, unusual, or unusually severe infections. Viral infections are particularly dangerous because T cells are the body’s primary defense against viruses. Pneumonia and chronic diarrhea are common early signs.
Yeast infections of the mouth (thrush) and diaper area occur frequently, as does pneumonia caused by Pneumocystis jirovecii, an opportunistic fungus that rarely causes illness in people with healthy immune systems. Children with CID often fail to gain weight and grow at expected rates, a pattern sometimes called failure to thrive. Some forms of CID also involve immune dysregulation, where the immune system attacks the body’s own tissues, leading to autoimmune symptoms like skin rashes, low blood counts, or organ inflammation.
How CID Is Diagnosed
Diagnosis relies on a combination of clinical suspicion and laboratory testing. When a child presents with recurrent or severe infections, blood tests can measure the number and types of immune cells present. Lymphocyte subset analysis counts T cells, B cells, and natural killer cells separately. In CID, T-cell counts often fall in a gray zone: not as devastatingly low as in SCID, but below normal for age. For context, T-cell counts between 0.05 and 1.0 × 10⁹ per liter fall in the range associated with leaky or atypical forms of severe disease, with the exact threshold depending on the child’s age.
Beyond counting cells, doctors test whether the T cells that are present actually work. Mitogen stimulation tests expose T cells to substances that should trigger them to multiply. A weak response confirms functional impairment even when cell numbers look reasonable. Genetic testing can then pinpoint the exact mutation responsible, which helps guide treatment decisions and genetic counseling for the family.
Treatment Options
The most definitive treatment for CID is a hematopoietic stem cell transplant (commonly called a bone marrow transplant). This replaces the faulty immune system with donor stem cells capable of producing functional T and B cells. For SCID specifically, five-year survival after transplant now exceeds 70% overall, and for babies transplanted before 3.5 months of age and before infections set in, survival reaches 80 to 95% regardless of the donor type used. Outcomes have improved dramatically over time, rising from 56% five-year survival before 1995 to the current figures.
Transplant outcomes vary by the specific genetic defect. In a large European study of patients transplanted between 1968 and 2005, those with certain genetic subtypes that preserve B cells had 70% ten-year survival, compared to 51% for subtypes in which B cells were also absent. Most deaths occur within the first two years after transplant, and long-term survivors generally do well.
While awaiting transplant, or for patients whose CID is less severe, supportive treatments help prevent infections. Immunoglobulin replacement therapy provides the antibodies the body cannot make on its own, delivered through regular infusions. After a successful transplant, many patients can eventually stop immunoglobulin therapy. In one long-term study, 63% of transplant recipients achieved independence from immunoglobulin supplementation.
For one specific form of combined immunodeficiency caused by adenosine deaminase deficiency, enzyme replacement therapy can serve as a bridge treatment while a transplant or gene therapy is being arranged. Gene therapy, which corrects the underlying genetic defect in a patient’s own cells, has become an option for select genetic subtypes and continues to expand as a treatment approach.
Living With CID
Without treatment, babies with the most severe forms of CID typically die from overwhelming infection within the first year of life. With early diagnosis and appropriate treatment, the outlook is far more favorable. The challenge lies in identifying CID early enough, since standard newborn screening misses most cases.
Parents of children with CID need to be vigilant about infections, particularly respiratory illnesses and exposures to live vaccines, which can cause serious disease in an immunocompromised child. Infection prevention measures, including careful hygiene and avoiding contact with sick individuals, are a daily reality until the immune system is restored through transplant or other definitive therapy. Children who achieve a successful transplant can often return to a largely normal life, attending school and participating in typical activities as their reconstituted immune system takes over.