Introduction To Primary Immune Deficiencies

Primary immune deficiency diseases comprise more than 450 distinct disorders. These conditions fundamentally impair the development or functional capacity of the immune system. Most of these disorders result from a single gene pathogenic variant. More complex inheritance patterns can also occur. Diagnostic delays frequently lead to irreversible end-organ damage or fatal outcomes. Prenatal diagnosis allows for early identification and optimal perinatal management.

Indications For Prenatal Diagnosis

Prenatal evaluation is strongly indicated when there is a known family history of a primary immune deficiency or when specific fetal anomalies are detected during routine obstetric care.

Family History

• A known proband in the family strongly indicates the need for targeted genetic testing.
• Identifying the specific genetic variant in the proband is the cornerstone for facilitating prenatal diagnosis in subsequent pregnancies.

Fetal Ultrasonography Findings

Advanced fetal ultrasound and fetal echocardiography can identify structural anomalies suggestive of underlying immune defects.

Genetic Counseling

Genetic counseling is a critical component of managing families with a recognized history of primary immune deficiency diseases.

Role Of The Physician

• Counseling is best performed by a physician who has direct knowledge of the clinical manifestations of the specific disease.
• Identifying a patient's specific genetic subgroup by DNA analysis is primarily useful for genetic counseling and prenatal diagnosis.
• This genetic identification is highly recommended for the first affected patient in a kindred.

Counseling In Severe Combined Immunodeficiency (SCID)

• Genetic counseling is the most compelling reason for genetic sequencing to identify the pathogenic gene variant in SCID.
• Early identification of the variant can lead to a significantly better transplant experience.
• It allows clinicians to avoid or reduce dosages of conditioning agents during hematopoietic stem cell transplantation.

Counseling In Calcium Channel Defects

• Genetic testing is strictly required to identify specific pathogenic variants in genes such as ORAI1 or STIM1.
• This identification directly aids in genetic counseling and guides prenatal diagnosis for future pregnancies.

Carrier Screening And Testing Modalities

Definitive prenatal diagnosis and carrier screening rely on targeted molecular genetics and specialized cellular assays.

Carrier Screening In Chronic Granulomatous Disease (CGD)

• The X-linked carrier state is usually easily diagnosed in the mother.
  • This is accomplished using the dihydrorhodamine flow cytometric assay.
  • The assay measures oxidant production through increased fluorescence.
  • Female carriers typically display a bimodal response to cellular stimulation.
• All possibly affected females should be tested by the dihydrorhodamine assay to exclude the carrier state.
• A subset of extremely lyonized carriers with less than 5% positive cells may experience chronic clinical problems.

DNA Analysis In Carriers

• Diagnosis by DNA analysis is strongly recommended in suspected carriers with a normal dihydrorhodamine test who are related to a known proband.
• A normal dihydrorhodamine test in an obligate carrier is a rare finding.
  • This may indicate that the affected patient has a spontaneous de novo mutation.
  • In such instances, the mother may not actually be a carrier of the disease.

Early Postnatal Testing And Newborn Screening

When targeted prenatal diagnosis is not performed, early postnatal testing serves as a critical safety net for identifying severe defects before the onset of symptomatic infections.

T-Cell Receptor Excision Circles (TRECs)

• Severe combined immunodeficiency is included in the newborn screening program using the T-cell receptor excision circles assay.
  • T-cell receptor excision circles are circular DNA fragments.
  • They are generated during the V(D)J rearrangement of the T-cell receptor variable region.
  • Because they do not replicate during cell division, they serve as an accurate biomarker.
  • They reliably identify low numbers of recent thymic emigrants.
• Quantitative polymerase chain reaction is used to detect these levels.

Kappa Recombination Excision Circles (KRECs)

• Kappa excision circles are generated during early B-cell development.
  • Assaying kappa excision circles simultaneously with TRECs expands the diagnostic yield of the newborn screen.
  • It allows for the early postnatal identification of infants with agammaglobulinemia.

Evaluating Maternal Engraftment

• Infants with severe combined immunodeficiency completely lack the capacity to reject foreign tissues.
  • Transplacental passage of maternal T-lymphocytes during pregnancy can result in maternal engraftment within the immunodeficient fetus.
  • Postnatally, this manifests with a phenotype closely resembling graft-versus-host disease.
  • The presentation is characterized by severe erythroderma, hepatosplenomegaly, and diarrhea.
• Fluorescence in situ hybridization for the X and Y chromosomes can definitively confirm maternal cell engraftment in male infants.

Prenatal Alloimmune Complications

Prenatal immune evaluations must also consider acquired alloimmune conditions that develop in utero.

Alloimmune Neonatal Neutropenia

• This condition occurs after the transplacental transfer of maternal alloantibodies.
  • These immunoglobulin G antibodies are directed against specific antigens on the fetal neutrophils.
  • Prenatal sensitization induces the production of these maternal antibodies.
• Infants typically present within the first two weeks of life with severe neutropenia and infections.
• The condition spontaneously resolves by approximately seven weeks of age as the maternal antibodies naturally decay.

Hemolytic Disease Of The Newborn

• Hemolytic disease of the newborn is a severe alloimmune condition developing in the fetus.
  • It is driven by maternal immunoglobulin G antibodies transferring via the placenta.
  • These antibodies attack and lyse the red blood cells within the fetal circulation.
• This destruction results in reticulocytosis and severe anemia.
• Fetal death may occur in utero due to progressive heart failure, a condition known as hydrops fetalis.