Algorithmic Approach to a Child with Suspected Immune Dysfunction

Clinical Recognition and Suspicion

• The initial diagnosis of primary immune deficiency diseases (PIDDs) is often delayed due to a rare individual disease incidence and a low index of suspicion, as symptoms can easily mimic common, non-specific childhood illnesses.
• Physicians must be acutely aware of clinical "red flags" that mandate a thorough immunologic evaluation, which include recurrent and/or sentinel infections, fever without a known focus, periodontitis, poor formation of pus at infection sites, and unusual inflammatory or autoimmune diseases.
• A high burden of infections in a child is the most common reason for initiating an immunologic workup.
• Evaluation is strongly warranted for patients with severe infections requiring hospitalization, prolonged or unsuccessful treatment with intravenous antibiotics, and infections caused by unusual or opportunistic pathogens.
• Adverse reactions to live attenuated virus vaccines, such as rotavirus, varicella, or measles-mumps-rubella (MMR), are critical warning signs of underlying immune dysfunction.
• A family history of PIDD or the presence of consanguinity should significantly increase clinical suspicion for an inherited immunologic defect.

Clinical Clues Guiding the Diagnostic Algorithm

• Clinical features can help guide the initial laboratory evaluation toward a specific compartment of the immune system.
• B-cell (humoral) defects are highly suggested by recurrent bacterial infections of the upper and lower respiratory tracts with encapsulated bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae), severe Giardia lamblia gastrointestinal infections, and reduced levels of immunoglobulins.
• T-cell (combined) defects are suspected when a child presents with systemic illness after vaccination with live viruses, chronic oral candidiasis after 6 months of age, failure to thrive, chronic diarrhea, absent lymph nodes or tonsils, and opportunistic infections like Pneumocystis jirovecii.
• Phagocyte or macrophage dysfunctions present with severe skin, liver, or lymph node abscesses, severe periodontitis, delayed umbilical cord separation, and recurrent infections with Staphylococcus, Aspergillus, or atypical mycobacteria.
• Complement deficiencies are classically characterized by recurrent sepsis or meningitis with encapsulated bacteria (Streptococcus, Pneumococcus, Neisseria) and the presence of early-onset autoimmune diseases like systemic lupus erythematosus.

Step 1: Initial Basic Screening (Level 1 Testing)

• The implementation of newborn screening for severe combined immunodeficiency (SCID) allows for the early detection of infants with very low T cells before symptoms arise.
• Newborn screening relies on the quantitative polymerase chain reaction (PCR) of T-cell receptor excision circles (TRECs), which identifies low numbers of recent thymic emigrants.
• In some regions, kappa excision circles (KRECs) are assayed simultaneously with TRECs to identify infants with severe B-cell defects like agammaglobulinemia.
• For symptomatic children or those not detected by newborn screening, basic screening begins with a complete blood count (CBC) with differential counts.
• The CBC aids in identifying lymphopenia, which is a hallmark of SCID, or neutropenia, which points toward congenital phagocytic disorders.
• The absolute lymphocyte count acts as a crucial starting place in the algorithm to suggest the type of SCID or cellular defect that may be present.
• A peripheral blood smear should be reviewed for specific findings, such as Howell-Jolly bodies (suggestive of asplenia) or giant granules (seen in Chédiak-Higashi syndrome).
• Quantitative serum immunoglobulin levels, specifically IgG, IgA, IgM, and IgE, must be measured and compared to age-matched and race-matched normal values.
• Antibody responses to prior routine vaccinations, such as diphtheria, tetanus, pneumococcus, and Haemophilus, should be assessed to evaluate specific functional antibody production.
• For the complement system, total hemolytic complement activity of the classical pathway (CH50) and alternative pathway (AH50) serve as the best initial screening tests.

Step 2: Targeted Pathway-Specific Evaluation (Level 2 Testing)

• If the initial screening tests reveal abnormalities, or if clinical suspicion remains high despite normal initial tests, the algorithm branches into specific evaluations based on the suspected defective arm of the immune system.

Evaluation of Suspected Antibody (Humoral) Deficiencies

• If basic screening shows low immunoglobulins, flow cytometry is essential to demonstrate the presence or absence of circulating B cells.
• Flow cytometry utilizes specific surface markers, such as CD19 or CD20, to accurately identify and enumerate B-cell populations.
• The complete absence of CD19+ B cells in the peripheral circulation strongly suggests congenital agammaglobulinemia, such as X-linked agammaglobulinemia (XLA).
• In patients over 2 years of age with normal total immunoglobulin levels but a history of recurrent sinopulmonary infections, specific antibody deficiency (SAD) should be evaluated.
• The gold standard for diagnosing SAD involves evaluating the antibody response to the 23-valent pneumococcal polysaccharide vaccine (PPV23).
• Protective antibody titers against pneumococcal polysaccharides are expressed in mg/mL, with a titer of 1.3 mg/mL generally considered protective against mucosal infections.
• Further detailed evaluation may include flow cytometry to enumerate the percentages of naive, memory, and switched memory B cells to diagnose conditions like Common Variable Immunodeficiency (CVID) or class switch defects.

Evaluation of Suspected Cell-Mediated (T-Cell and Combined) Deficiencies

• When a T-cell or combined immunodeficiency is suspected, flow cytometry is utilized to precisely enumerate T cells and their specific subsets.
• Markers such as CD3 (total T cells), CD4 (T-helper cells), and CD8 (cytotoxic T cells) are used to quantify T-lymphocyte populations, while CD16 and CD56 are used to quantify Natural Killer (NK) cells.
• The ratio of CD4+ to CD8+ T cells is calculated, as significant alterations in this ratio (normally approximately 2:1) can indicate immunodeficiency.
• Functional assays for T-cell activity are conducted via cell culture studies, where isolated T-cells are stimulated with specific antigens (like tetanus toxoid) or mitogens such as phytohemagglutinin (PHA) or concanavalin.
• A proliferative response to PHA that is less than 10% of a normal control is a critical diagnostic threshold confirming a severe combined immunodeficiency.
• The measurement of specific T-cell isoforms, such as the differentiation between naive T cells (CD45RA) and memory T cells (CD45RO), helps identify conditions like maternal engraftment or Omenn syndrome.
• Advanced in vitro tests include the measurement of T-cell cytokine production by ELISPOT assay or assessing intracellular phosphorylation events following cytokine stimulation.

Evaluation of Suspected Phagocytic Defects

• When deep tissue abscesses or infections with catalase-positive organisms suggest a phagocyte defect, the evaluation focuses on neutrophil counts and functional assays.
• If Chronic Granulomatous Disease (CGD) is suspected, the diagnosis is most often confirmed by performing flow cytometry using dihydrorhodamine (DHR) to measure oxidant production.
• The DHR assay measures the oxidative burst through increased fluorescence when the dye is oxidized by hydrogen peroxide, largely replacing the older nitroblue tetrazolium (NBT) slide test.
• In cases where severe MPO deficiency causes a falsely positive DHR result for neutrophils, the evaluation of eosinophils (which still reduce DHR in MPO deficiency but not in CGD) can differentiate the two conditions.
• If a motility or adhesion defect is suspected, such as Leukocyte Adhesion Deficiency (LAD) type 1, flow cytometry is used to evaluate the surface expression of CD11b and CD18 (beta-2 integrins).
• LAD type 1 is characterized by the absence or severe reduction of CD11b/CD18 on both stimulated and unstimulated neutrophils.
• LAD type 2 is distinctly diagnosed by flow cytometric measurement demonstrating the absence of sialyl Lewis X (CD15) on the surface of neutrophils.

Evaluation of Suspected Complement Deficiencies

• A negative or abnormally low CH50 (classical pathway) or AH50 (alternative pathway) screening assay mandates further immunochemical tests to define the precise defect.
• Techniques such as radial immunodiffusion, enzyme-linked immunosorbent assay (ELISA), or nephelometry are used to quantify individual complement components like C3, C4, and properdin.
• Functional activity assays of single components can be performed by mixing patient plasma with complement-depleted serum to check if hemolytic activity is restored by a specific purified protein.
• The measurement of specific complement activation products (e.g., C3d, C4d, Ba, Bb, and the soluble terminal complement complex sC5b-9) helps distinguish primary genetic defects from acquired complement consumption seen in active immune complex diseases.
• Flow cytometry is the standard technique for diagnosing paroxysmal nocturnal hemoglobinuria (PNH) by detecting reduced surface levels of complement regulators CD55 (decay-accelerating factor) and CD59.

Step 3: Advanced Molecular and Genetic Testing (Level 3 Testing)

• The most definitive method of diagnosis for the vast majority of primary immunodeficiency disorders is targeted gene sequencing.
• Gene sequencing is frequently performed by requesting a specific SCID gene panel, a more extensive primary immunodeficiency (PID) gene panel, or through whole exome or whole genome sequencing.
• Identification of specific pathogenic gene variants is essential for providing accurate genetic counseling, allowing for prenatal diagnosis, and predicting the clinical prognosis of the disease.
• Genetic diagnosis is particularly crucial because specific variants dictate the feasibility and conditioning requirements for definitive treatments like hematopoietic stem cell transplantation (HSCT) or gene therapy.
• In patients presenting with primary immune regulatory disorders (PIRDs), which feature severe early-onset autoimmunity or enteropathy, next generation sequencing is strictly necessary to establish a genetic diagnosis and guide the use of targeted biological therapies.