Introduction And Clinical Red Flags

Primary immune deficiency diseases encompass over 450 distinct disorders that impair the immune system.
Diagnosis is frequently delayed due to a low index of clinical suspicion.
Clinical screening relies on identifying specific warning signs or "red flags".
These red flags include recurrent or sentinel infections, and fever without a clear focus.
Severe periodontitis and poor formation of pus at infection sites are major warning signs.
Unusual autoimmune or inflammatory conditions mandate further screening.
Patients often experience severe infections requiring hospitalization and prolonged intravenous antibiotics.
Infections caused by opportunistic pathogens or live attenuated vaccines strongly suggest an underlying immunodeficiency.

Defect-Specific Clinical Screening Patterns

Clinical suspicion is specifically guided by distinct patterns of infection.

Defect Compartment	Characteristic Clinical Screening Patterns
Antibody (B-cell)	Characterized by recurrent bacterial sinopulmonary infections. Common pathogens include Streptococcus pneumoniae and Haemophilus influenzae. Susceptibility to gastrointestinal infections like Giardia and enteroviruses is prominent.
Cell-mediated (T-cell)	Suggested by severe opportunistic infections with Pneumocystis jirovecii or Mycobacterium avium-intracellulare. Features include severe viral infections, chronic mucocutaneous candidiasis, and severe failure to thrive.
Phagocytic	Manifests as recurrent skin, liver, and lymph node abscesses. Common pathogens include Staphylococcus aureus, gram-negative bacteria, and fungi. Severe periodontitis and poor wound healing are classic.
Complement	Clinically screened when patients present with early-onset systemic lupus erythematosus. Features recurrent sepsis and meningitis caused by blood-borne encapsulated bacteria like Neisseria and Pneumococcus.

Newborn screening programs facilitate the early detection of Severe Combined Immunodeficiency (SCID).
Early detection allows for potentially curative treatment before the onset of symptomatic infections.
The primary screening method utilizes a quantitative polymerase chain reaction assay.
This assay specifically measures T-cell receptor excision circles (TRECs).
TRECs are episomal DNA byproducts generated during the V(D)J rearrangement of T-cell receptor chains.
They do not replicate during cell division.
Therefore, they serve as an accurate biomarker for quantifying recent thymic emigrants.
A low or absent TREC count on a newborn screen raises high suspicion for SCID.
This abnormal result necessitates prompt evaluation of lymphocyte subsets and functional testing.
In certain regions, kappa excision circles (KRECs) are assayed simultaneously with TRECs.
KRECs are generated during early B-cell development.
Assaying KRECs identifies infants with profound B-cell defects such as congenital agammaglobulinemia.

The initial laboratory evaluation must begin with broadly available screening tools.

Evaluation begins with a complete blood count with differential.
This identifies critical baseline abnormalities such as persistent lymphopenia, neutropenia, or marked leukocytosis.
A peripheral blood smear should be reviewed for distinct morphological clues.
The presence of giant lysosomal granules is characteristic of Chédiak-Higashi syndrome.
Howell-Jolly bodies on a smear are indicative of isolated congenital asplenia.

Evaluation Type	Specific Laboratory Methodologies
Humoral (B-cell) Screening	Quantitative measurement of serum immunoglobulin levels, specifically IgG, IgA, IgM, and IgE. Assessment of specific antibody production by measuring titers in response to protein vaccines like diphtheria and tetanus. Assessing titers against polysaccharide vaccines like pneumococcus. Flow cytometry is used to enumerate circulating B-cells using surface markers CD19 and CD20.
Cellular (T-cell) Screening	Flow cytometry accurately quantifies total T-cells, helper T-cells (CD4), and cytotoxic T-cells (CD8). Natural killer cells are also quantified. Analysis of naive versus memory T-cell populations is performed by evaluating CD45 isoform expression. Functional screening utilizes lymphocyte proliferation assays to evaluate responses to mitogens like phytohemagglutinin.

The primary screen for Chronic Granulomatous Disease evaluates the neutrophil oxidative burst.
This is performed using the dihydrorhodamine reduction assay by flow cytometry or the older nitroblue tetrazolium slide test.
Flow cytometry evaluates the surface expression of adhesion molecules, such as CD11b and CD18, for screening leukocyte adhesion deficiency.
The functional integrity of the classical complement pathway is screened using the total hemolytic complement assay (CH50).
The AH50 assay is utilized to screen the functional activity of the alternative complement pathway.
If CH50 or AH50 screening results are abnormal, further immunochemical quantification of single complement components is strictly indicated.

Definitive diagnosis is established through genetic mutation analysis.
Genetic screening follows abnormal functional or quantitative laboratory screening.
It utilizes targeted primary immunodeficiency gene panels.
Broader modalities include whole-exome sequencing or whole-genome sequencing.