Immunodeficiency Syndromes

• Definition: Immunodeficiency syndromes are disorders in which one or more arms of host defense — antibody, T cell, phagocyte, or complement — fail, producing infections that are too frequent, too severe, too persistent, or caused by organisms of unusually low virulence.
• Primary (inborn errors of immunity): monogenic or chromosomal defects of immune development. Several hundred are now catalogued by the International Union of Immunological Societies, but a handful dominate exams: selective IgA deficiency, 22q11.2 deletion (DiGeorge), X-linked agammaglobulinemia, SCID, chronic granulomatous disease, and terminal complement deficiency.
• Secondary (acquired): far more common in practice — HIV, hematologic malignancy, chemotherapy, glucocorticoids, biologics (rituximab, TNF inhibitors), protein-losing states, asplenia, malnutrition.

Epidemiology worth recalling

• Antibody defects predominate: humoral/B-cell defects account for the majority of primary immunodeficiency diagnoses; selective IgA deficiency is the most frequently identified primary immune defect and is usually clinically silent, whereas 22q11.2 deletion is the most common chromosomal microdeletion syndrome and the classic congenital defect of T-cell development.
• Sex skew: several severe forms are X-linked and therefore essentially male-only — Bruton agammaglobulinemia (BTK), the common gamma-chain form of SCID (IL2RG), Wiskott-Aldrich, and the majority of chronic granulomatous disease (gp91phox).
• Age at onset: SCID and complete DiGeorge present in the first weeks to months; B-cell defects surface after ~6 months as maternal IgG wanes; common variable immunodeficiency characteristically declares itself in adolescence or adulthood.
• Why it matters: untreated SCID is uniformly fatal in infancy, which is why the TREC (T-cell receptor excision circle) assay was added to the U.S. Recommended Uniform Screening Panel by the HHS Advisory Committee on Heritable Disorders in Newborns and Children and is now performed in all states. Early detection converts a lethal disease into a curable one, and delayed recognition of antibody deficiency leads to irreversible bronchiectasis.

Primary — grouped by compartment (all non-modifiable)

• B-cell/antibody defects: BTK loss-of-function (X-linked agammaglobulinemia) blocks pre-B receptor signaling; class-switch recombination defects (CD40 ligand, AID) cause hyper-IgM; polygenic/unknown defects underlie common variable immunodeficiency and selective IgA deficiency.
• T-cell and combined defects: IL2RG (X-linked, T–B+NK–), ADA deficiency (T–B–NK–, toxic metabolite accumulation), RAG1/RAG2 or Artemis (T–B–NK+, failed V(D)J recombination), JAK3, IL7RA.
• Developmental/syndromic: 22q11.2 hemizygous deletion disrupts third and fourth pharyngeal pouch development; WAS, ATM (ataxia-telangiectasia), STAT3 dominant-negative (hyper-IgE/Job syndrome).
• Phagocyte defects: NADPH oxidase subunit mutations (chronic granulomatous disease), ITGB2 (leukocyte adhesion deficiency type 1), LYST (Chédiak-Higashi).
• Complement defects: C1-C4 (immune-complex/lupus-like disease), C3 (pyogenic infection), C5-C9 and properdin (Neisseria).

Secondary — largely modifiable or iatrogenic

• Infection: HIV depletes CD4+ T cells; measles and EBV cause transient immune suppression.
• Drugs: chronic systemic glucocorticoids, cytotoxic chemotherapy, calcineurin inhibitors, rituximab (prolonged hypogammaglobulinemia), TNF inhibitors (granulomatous defense → tuberculosis reactivation), eculizumab (functional terminal complement deficiency → meningococcus).
• Protein loss: nephrotic syndrome, protein-losing enteropathy, extensive burns.
• Anatomic/structural: splenectomy or functional asplenia in sickle cell disease → encapsulated organism sepsis; indwelling catheters; malnutrition; uncontrolled diabetes.

Risk factors examiners plant in the stem

• Consanguinity or a maternal family history of infant male deaths — points to autosomal recessive or X-linked disease.
• Live vaccine administration followed by disseminated infection (rotavirus, BCG, oral polio) — unmasks SCID.
• Persistent thrush beyond infancy, delayed umbilical cord separation, or an absent thymic shadow on infant chest radiograph.

• Maternal IgG is the clock: transplacental IgG peaks at term and decays over roughly the first half-year. A pure antibody defect is therefore masked until maternal IgG falls, whereas a T-cell defect — which maternal antibody cannot substitute for — presents within weeks.
• X-linked agammaglobulinemia: BTK transduces pre-B cell receptor signals. Without it, development arrests at the pro-B/pre-B stage, so circulating CD19+ B cells are essentially absent, all immunoglobulin isotypes are low, and germinal centers/tonsils/adenoids fail to form — hence absent tonsils and scant lymphoid tissue on exam. Opsonization of encapsulated bacteria fails, giving recurrent otitis, sinusitis, and pneumonia with S. pneumoniae and H. influenzae, plus a peculiar vulnerability to enteroviral meningoencephalitis and Giardia.
• 22q11.2 deletion: failure of third and fourth pharyngeal pouch derivatives yields thymic hypoplasia (variable T-cell lymphopenia), parathyroid hypoplasia (hypocalcemia → tetany and seizures), conotruncal cardiac anomalies (truncus arteriosus, tetralogy, interrupted aortic arch), and palatal/facial dysmorphism. Most patients are partial DiGeorge with enough thymic tissue for near-normal immunity; complete athymia behaves like SCID.
• SCID: whatever the gene, the common endpoint is absent functional T cells. Because T-cell help is required for B-cell class switching, humoral immunity fails even when B cells are numerically present (T–B+ phenotypes) — which is why quantitative IgG cannot exclude SCID. Absent cellular immunity permits Pneumocystis jirovecii, CMV, chronic rotavirus, candidiasis, and disseminated live-vaccine organisms, and permits engraftment of transfused or transplacentally acquired maternal lymphocytes → graft-versus-host disease.
• Chronic granulomatous disease: NADPH oxidase cannot generate superoxide, so phagocytes ingest but cannot kill. Catalase-positive organisms degrade their own hydrogen peroxide, denying the phagocyte a substrate; persistent viable organisms drive granuloma formation with obstructive mass effects.
• Terminal complement deficiency: the membrane attack complex is the principal defense against Neisseria, whose thin cell wall requires lysis rather than phagocytosis alone.

The infant who fails to thrive with infection

• SCID: within the first months — intractable oral thrush, chronic watery diarrhea (rotavirus, often vaccine-strain), interstitial pneumonitis from Pneumocystis or CMV, and failure to thrive. Physical exam is notable for what is missing: no palpable lymph nodes, no tonsils, and no thymic shadow on chest radiograph. An erythrodermic rash with eosinophilia and hepatosplenomegaly suggests Omenn syndrome or maternal T-cell engraftment.
• 22q11.2 deletion: neonatal hypocalcemic tetany or seizures, a murmur or cyanosis from a conotruncal defect, cleft palate or velopharyngeal insufficiency with hypernasal speech, and dysmorphic facies (hypertelorism, low-set ears, micrognathia). Older children present with learning disability and, notably, markedly elevated risk of schizophrenia and other psychiatric illness.
• X-linked agammaglobulinemia: a boy who was well until about 6 months, then recurrent otitis media, sinusitis, and pneumonia; absent tonsils and non-palpable lymph nodes despite active infection. Enteroviral meningoencephalitis and *Giardia*-associated chronic diarrhea are the classic non-pyogenic clues.

Pattern-recognition cues by compartment

• Phagocyte defect: deep abscesses (liver, lymph node), osteomyelitis, Serratia and Aspergillus, delayed separation of the umbilical cord with leukocytosis and no pus (leukocyte adhesion deficiency), poor wound healing.
• Complement defect: recurrent or recurrent-familial meningococcal disease, often in an adolescent, or an angioedema/lupus-like phenotype with early-classical-pathway deficiency.
• Secondary immunodeficiency: the stem names the exposure — HIV risk behavior, recent rituximab or high-dose steroids, splenectomy, or nephrotic-range proteinuria.

Screening framework

• The Jeffrey Modell Foundation 10 Warning Signs are the widely taught trigger for evaluation: multiple pneumonias or ear infections in a year, two or more months of antibiotics with little effect, recurrent deep abscesses, persistent thrush after age one, need for intravenous antibiotics to clear infection, two or more deep-seated infections, failure to thrive, and a family history of primary immunodeficiency.

Step 1 — universal newborn screen

• TREC assay on the newborn dried blood spot detects T-cell receptor excision circles, a marker of recent thymic emigrants. Low or absent TRECs flag SCID and severe T-cell lymphopenia before infection occurs; this is on the HHS-endorsed Recommended Uniform Screening Panel and is performed in all U.S. states. An abnormal screen mandates urgent confirmatory flow cytometry, not outpatient follow-up.

Step 2 — inexpensive first-line labs

• CBC with manual differential: an absolute lymphocyte count that is low for age in an infant is the single most useful screening abnormality for SCID; neutropenia, thrombocytopenia with small platelets (Wiskott-Aldrich), or Howell-Jolly bodies (asplenia) redirect the workup.
• Quantitative IgG, IgA, IgM, IgE interpreted against age-specific norms.
• Chest radiograph for absent thymic shadow.

Step 3 — compartment-directed confirmation

• Lymphocyte flow cytometry for CD3/CD4/CD8, CD19, and CD16/56 defines the SCID phenotype (T–B+NK–, T–B–NK+, T–B–NK–) and shows absent CD19+ B cells with normal T cells in X-linked agammaglobulinemia.
• Functional antibody testing: specific IgG titers to tetanus/diphtheria (protein) and pneumococcal polysaccharide before and after vaccination — failure to mount titers proves a functional defect even when total IgG is borderline.
• T-cell function: mitogen (PHA) proliferation assays.
• Complement: CH50 screens the classical pathway and terminal components (undetectable in C5-C9 deficiency); AH50 screens the alternative pathway.
• Phagocytes: dihydrorhodamine 123 flow cytometry is the current standard for chronic granulomatous disease and has largely replaced the classic nitroblue tetrazolium slide test; CD18 flow cytometry for leukocyte adhesion deficiency.
• Genetics: chromosomal microarray or FISH for 22q11.2; targeted gene panel or exome sequencing confirms BTK, IL2RG, ADA, RAG1/2.
• Always exclude HIV with a fourth-generation antigen/antibody immunoassay, and use HIV nucleic acid testing in infants because of maternal antibody. Common variable immunodeficiency is a diagnosis of exclusion using the ESID criteria: low IgG with low IgA and/or IgM, poor vaccine response, onset after age four, and no other identifiable cause.

Immediate protective measures once a severe T-cell defect is suspected

• Stop all live attenuated vaccines — rotavirus, BCG, MMR, varicella, live attenuated influenza, oral polio — per ACIP and the IDSA 2013 guideline for vaccination of the immunocompromised host. Household contacts should also avoid oral polio vaccine.
• Transfuse only irradiated, leukoreduced, CMV-safe cellular blood products to prevent transfusion-associated graft-versus-host disease and CMV transmission.
• Protective isolation and avoidance of breastfeeding if the mother is CMV-seropositive (practice varies).
• Antimicrobial prophylaxis: trimethoprim-sulfamethoxazole for Pneumocystis, plus antifungal coverage (an azole such as fluconazole) in SCID.

Definitive therapy

• SCID: allogeneic hematopoietic stem cell transplantation is curative and outcomes are best when performed in the first few months of life, before infection is established. ADA-deficient SCID additionally has enzyme replacement (pegylated adenosine deaminase) as a bridge, and ex vivo autologous gene therapy is an established alternative in specialized centers.
• Complete DiGeorge/athymia: allogeneic cultured thymus tissue implantation is FDA-approved; partial DiGeorge needs no immune reconstitution.
• Chronic granulomatous disease: lifelong trimethoprim-sulfamethoxazole plus an anti-mold azole (itraconazole) and interferon-gamma; HSCT is curative.

Ongoing/supportive therapy

• Immunoglobulin replacement (IVIG or SCIg) for antibody deficiency, dosed to clinical response and trough IgG; it does not replace T-cell function.
• 22q11.2 deletion: calcium and calcitriol for hypoparathyroidism, cardiac surgical repair, palate repair, developmental and psychiatric follow-up.
• Complement deficiency and eculizumab recipients: meningococcal ACWY and serogroup B vaccination with boosters, per ACIP; asplenia additionally requires pneumococcal and H. influenzae type b coverage.

Contraindicated

• Live vaccines and non-irradiated cellular blood products in severe T-cell deficiency.
• IgA-containing blood products in IgA-deficient patients with anti-IgA antibodies — use washed or IgA-depleted products.
• Empiric high-dose immunosuppression before immunodeficiency has been excluded.

Disease-related

• Bronchiectasis and chronic sinus disease: the signature late complication of untreated or under-treated antibody deficiency; repeated neutrophilic airway inflammation destroys the bronchial wall. Signalled by chronic productive cough with tram-track or signet-ring findings on high-resolution CT.
• Overwhelming sepsis with encapsulated organisms in antibody deficiency and asplenia — an emergency requiring immediate empiric broad-spectrum antibiotics before workup is complete.
• Fulminant meningococcemia in terminal complement deficiency; recurrence is characteristic and mandates vaccination and family screening.
• Disseminated vaccine-strain infection after a live vaccine in undiagnosed SCID (BCG-osis, vaccine-associated paralytic polio, chronic rotavirus) — often the presenting event.
• Hypocalcemic seizure or laryngospasm in 22q11.2 deletion — an emergency; check ionized calcium in any neonate with a conotruncal defect and a seizure.
• Autoimmunity and malignancy: immune dysregulation, not just infection. CVID brings autoimmune cytopenias, granulomatous-lymphocytic interstitial lung disease, and a markedly increased risk of non-Hodgkin lymphoma and gastric carcinoma; ataxia-telangiectasia and Wiskott-Aldrich carry high lymphoid malignancy risk; early classical complement deficiency produces a lupus-like syndrome.
• Obstructive granulomas of the gastrointestinal and genitourinary tract in chronic granulomatous disease.
• Enteroviral meningoencephalitis in X-linked agammaglobulinemia — indolent, progressive, and often fatal.

Treatment-related

• Transfusion-associated graft-versus-host disease from non-irradiated cellular products in severe T-cell deficiency — rash, diarrhea, hepatitis, and marrow aplasia; nearly uniformly fatal, an emergency to prevent, not to treat.
• Anaphylaxis to IgA-containing products in IgA-deficient patients with anti-IgA IgE — treat with intramuscular epinephrine 0.3 mg.
• Immunoglobulin infusion reactions: headache and aseptic meningitis, thromboembolism, hemolysis from isohemagglutinins, and renal dysfunction; slowing the infusion rate mitigates most.
• Post-HSCT: graft failure, graft-versus-host disease, conditioning toxicity, and incomplete B-cell reconstitution requiring continued immunoglobulin replacement.

• Timing localizes the lesion: infection from the first weeks of life with thrush, diarrhea, and failure to thrive = T-cell/combined defect; onset around 6 months, as maternal IgG wanes, with pyogenic sinopulmonary infection = B-cell defect. This is the single most testable discriminator.
• Absent tonsils and non-palpable lymph nodes in a boy with recurrent otitis and pneumonia is X-linked agammaglobulinemia — confirm with flow cytometry showing absent CD19+ B cells with normal T cells.
• Abnormal newborn TREC screen → next best step is lymphocyte flow cytometry, not immunoglobulin levels. Normal IgG never excludes SCID, because T–B+ phenotypes exist and maternal IgG is still circulating.
• Neonatal seizure + murmur + hypocalcemia = 22q11.2 deletion; confirm with FISH or chromosomal microarray, and remember the psychiatric association (schizophrenia) that examiners like to test in the adolescent version of the stem.
• Catalase-positive organisms (S. aureus, Serratia, Burkholderia, Nocardia, Aspergillus) with deep abscesses = chronic granulomatous disease; dihydrorhodamine 123 flow cytometry is the modern confirmatory test — nitroblue tetrazolium is the classic but superseded one.
• **Recurrent Neisseria meningitis = terminal complement (C5-C9) deficiency; screen with CH50** and vaccinate against meningococcus ACWY and B per ACIP. The same risk applies iatrogenically to patients on eculizumab.
• Two things never to do in suspected severe T-cell deficiency: give a live vaccine, or transfuse non-irradiated cellular blood. Either can kill the patient.
• Common distractor: recurrent infections in a daycare-attending toddler with normal growth and normal exam is usually normal immunity, not immunodeficiency — the discriminators are failure to thrive, unusual organisms, deep or invasive sites, and failure to clear with appropriate antibiotics.

• Primary immunodeficiencies: genetic defects in immune development; present early in life
• Secondary immunodeficiencies: acquired (HIV, malignancy, medications); more common in clinical practice
• DiGeorge syndrome (22q11 deletion): most common primary immunodeficiency; T-cell defect (thymic hypoplasia)
• X-linked agammaglobulinemia (Bruton): absent B cells → no immunoglobulins; recurrent bacterial infections after 6 months of age
• Severe combined immunodeficiency (SCID): defective T cells ± B/NK cells; "bubble boy disease"; death within months without treatment

"Recurrent infections + failure to thrive in an infant"

• Recurrent bacterial infections (S. pneumoniae, H. influenzae) → B-cell/antibody defects
• Recurrent viral/fungal/PCP → T-cell defects
• Recurrent Neisseria infections → complement defects (C5-C9)
• Recurrent catalase-positive organisms (S. aureus, Aspergillus, Serratia) → chronic granulomatous disease
• Opportunistic infections (PCP, CMV, toxoplasmosis) → severe T-cell defect (AIDS-defining)

Mnemonic for phagocytic defects: "NAILING"

• N-NADPH oxidase (CGD)
• A-Adhesion defects (leukocyte adhesion deficiency)
• I-Immunoglobulin opsonization defects
• L-Leukopenia
• I-Inflammatory cascade (complement)
• N-Neutrophil granule defects (myeloperoxidase)
• G-Glucose-6-phosphatase deficiency (rarely relevant)

1. 1.Confusing timing with immune compartment: Infections after 6 months = B-cell defect (maternal IgG waning); infections from birth = T-cell or complement defect. DiGeorge (T-cell) presents earlier than X-linked agammaglobulinemia (B-cell).

1. 1.Missing IgA deficiency complications: Often asymptomatic and found incidentally, but transfusion of IgA-containing blood products causes anaphylaxis. Screen before transfusion in patients with recurrent infections.

1. 1.Overdiagnosing immunodeficiency: Recurrent infections ≠ primary immunodeficiency until proven. Most children with frequent infections have normal immune systems; consider poor hygiene, daycare exposure, or frequent viral URIs before extensive workup.

• B-cell defects (agammaglobulinemia, IgG subclass): Intravenous immunoglobulin (IVIG) or subcutaneous immunoglobulin (SCIg)
• T-cell defects (DiGeorge, SCID): **Hema