Residency · Residency · Allergy Immunology

Primary Immunodeficiency - Humoral Deficiencies

Overview

Humoral (antibody) deficiencies represent the most common category of primary immunodeficiencies, accounting for approximately 50 to 65 percent of all identified PIDs. The spectrum ranges from selective IgA deficiency, the mildest and most common form, to X-linked agammaglobulinemia, one of the most severe. The clinical hallmark of humoral immunodeficiency is recurrent sinopulmonary infections with encapsulated bacteria, particularly Streptococcus pneumoniae and Haemophilus influenzae. Patients with antibody deficiency are also susceptible to enteroviral infections, Giardia lamblia, and Mycoplasma species. Warning signs in adults that should prompt evaluation include two or more pneumonias within 12 months, chronic sinusitis unresponsive to appropriate treatment, recurrent need for intravenous antibiotics, bronchiectasis, and a family history of primary immunodeficiency.

X-Linked Agammaglobulinemia (XLA/Bruton's Disease)

Genetics and Pathophysiology

X-linked agammaglobulinemia results from mutations in the BTK (Bruton tyrosine kinase) gene located on chromosome Xq21.3-22. BTK is a cytoplasmic tyrosine kinase essential for signaling through the pre-B cell receptor at the critical transition from pro-B to pre-B cell stage of B cell development. In the absence of functional BTK, B cell development arrests at the pro-B cell stage, resulting in absent or severely reduced mature B cells (less than 1 to 2 percent of lymphocytes). As an X-linked recessive disorder, it affects males, while carrier females are asymptomatic due to skewed X-inactivation favoring the functional allele in their B cell compartment. More than 600 distinct BTK mutations have been described, including missense, nonsense, frameshift, and splice site variants.

Clinical Features

Clinical manifestations typically begin at 6 to 9 months of age, when passively transferred maternal IgG wanes below protective levels. The presentation is dominated by recurrent sinopulmonary infections including pneumonia, sinusitis, and otitis media, caused predominantly by encapsulated organisms. Enteroviral meningoencephalitis, caused by echoviruses and coxsackieviruses, is a chronic and often fatal complication unique to agammaglobulinemic patients who cannot mount protective antibody responses to these viruses. Giardia lamblia causes chronic diarrhea, and Mycoplasma and Ureaplasma species may cause arthritis and urethritis. Physical examination characteristically reveals absent or very small tonsils and lymph nodes, reflecting the absence of B cell follicles, a finding that distinguishes XLA from CVID, in which lymphoid hyperplasia may be prominent.

Diagnosis

The diagnostic evaluation reveals markedly reduced or absent immunoglobulins of all classes (IgG, IgA, IgM, and IgE). B cells, identified by CD19+ or CD20+ markers, are absent or below 2 percent of lymphocytes. T cell numbers and function are normal, as are natural killer cells. BTK protein expression can be assessed by flow cytometry using intracellular staining of monocytes (which also express BTK), with absent staining confirming the diagnosis. Genetic testing for BTK mutations provides definitive confirmation.

Treatment

Treatment requires lifelong immunoglobulin replacement therapy, administered intravenously (IVIG) or subcutaneously (SCIG), targeting trough IgG levels of at least 500 mg/dL, with some experts advocating for targets of 800 mg/dL or higher for optimal infection prevention. Aggressive antibiotic treatment of breakthrough infections is essential. Live vaccines, particularly oral polio vaccine (due to the risk of vaccine-associated paralytic poliomyelitis), must be avoided. Patients should be monitored for enteroviral infections with cerebrospinal fluid surveillance if neurologic symptoms develop. With early diagnosis and adequate immunoglobulin replacement, prognosis is good and life expectancy approaches normal.

<image>A diagram illustrating B cell developmental arrest in XLA compared to normal B cell development. Two parallel tracks: Top track (normal): HSC to CLP to pro-B to pre-B (with pre-BCR shown) to immature B to mature B cell to plasma cell - fully mature pathway. Bottom track (XLA): HSC to CLP to pro-B then a large red "X" at the pro-B to pre-B transition where BTK is required for pre-BCR signaling. BTK shown as a molecule at the block point. Result: no mature B cells, no plasma cells, no immunoglobulins. Adjacent clinical features panel showing: absent tonsils (photograph-style illustration of open mouth with no visible tonsils), very low/absent immunoglobulin levels (bar graph with all isotypes near zero), and typical infections (S. pneumoniae, enteroviruses, Giardia). Flow cytometry plot showing absent CD19+ B cells vs normal control.</image>

Common Variable Immunodeficiency (CVID)

Definition and Epidemiology

Common variable immunodeficiency is the most clinically significant primary immunodeficiency, with a prevalence of approximately 1 in 25,000 to 1 in 50,000. It encompasses a heterogeneous group of disorders united by the shared features of hypogammaglobulinemia and impaired antibody responses. The diagnostic criteria established by ESID and ICON require IgG significantly below age-matched reference values (typically below 450 to 500 mg/dL), low IgA and/or low IgM, demonstrably poor specific antibody responses to vaccines (pneumococcal and tetanus), exclusion of other causes of hypogammaglobulinemia, and an age greater than 4 years (to exclude transient hypogammaglobulinemia of infancy).

Genetic Landscape

A monogenic cause is identified in approximately 10 to 25 percent of CVID patients with current genetic testing capabilities. The most commonly identified gene is TNFRSF13B, encoding TACI (transmembrane activator and CAML interactor), which harbors mutations in 8 to 10 percent of CVID patients. However, TACI mutations are also found in 1 to 2 percent of healthy controls, indicating incomplete penetrance and the requirement for additional genetic or environmental modifiers. Other identified genes include TNFRSF13C (BAFF receptor), ICOS, CD19, CD81, CD20, CD21, LRBA, CTLA4, NFKB1, NFKB2, PIK3CD (gain-of-function), PIK3R1, STAT3 (gain-of-function), and IKAROS (IKZF1).

Identification of the underlying genetic defect has direct therapeutic implications in several instances. CTLA-4 haploinsufficiency and LRBA deficiency are particularly important to identify because they are treatable with abatacept (CTLA-4-Ig fusion protein). NFKB1 haploinsufficiency has been recognized as the most common autosomal dominant monogenic cause of CVID.

Clinical Phenotypes

CVID manifests through several distinct clinical phenotypes that frequently overlap. The infection-only phenotype, affecting approximately 70 percent of patients, presents with recurrent sinopulmonary infections (pneumonia, sinusitis, bronchitis) and, if inadequately treated, progressive bronchiectasis. Gastrointestinal infections with Giardia, Campylobacter, and Salmonella are also common.

The polyclonal lymphocytic infiltration phenotype, affecting 10 to 20 percent of patients, is characterized by granulomatous-lymphocytic interstitial lung disease (GLILD), which manifests as granulomas and lymphocytic infiltrates in the lungs with CT findings of ground-glass opacities, nodules, and mediastinal lymphadenopathy. Additional features include granulomatous hepatitis, splenomegaly, and nodular regenerative hyperplasia of the liver.

The autoimmune phenotype, present in 25 to 30 percent of patients, most commonly manifests as autoimmune cytopenias: immune thrombocytopenia (ITP) in 10 to 20 percent, autoimmune hemolytic anemia (AIHA) in 5 to 10 percent, and Evans syndrome. Autoimmune cytopenias may precede the diagnosis of CVID by years and, when present in the context of CVID, are associated with an increased risk of lymphoma.

Malignancy risk is elevated 5 to 8 times above the general population, primarily consisting of non-Hodgkin lymphoma (especially marginal zone lymphoma) and gastric cancer. CVID-associated enteropathy, characterized by villous atrophy and nodular lymphoid hyperplasia, mimics celiac disease clinically but is distinguished by negative anti-tissue transglutaminase antibodies.

Diagnosis

Serum immunoglobulin quantification reveals low IgG with frequently low IgA and/or IgM. B cells are present (distinguishing CVID from XLA) but may be reduced in number. B cell subset analysis by flow cytometry provides important prognostic information: switched memory B cells (CD27+IgD-IgM-) are reduced below 2 percent of B cells in most CVID patients and correlate with autoimmunity and granulomatous disease. Plasmablasts (CD27++CD38++) are also reduced.

Specific antibody response testing is essential and is performed by measuring pre- and post-vaccination pneumococcal serotype titers following administration of Pneumovax 23 (unconjugated polysaccharide vaccine). Titers to at least 12 to 14 serotypes are measured 4 to 6 weeks after vaccination, with an inadequate response defined as less than 70 percent of tested serotypes reaching protective levels (at least 1.3 mcg/mL). Some patients respond to conjugate vaccine (Prevnar) but not to polysaccharide vaccine, reflecting a specific defect in T-independent B cell responses.

Secondary causes of hypogammaglobulinemia must be excluded, including medications (rituximab, anticonvulsants, corticosteroids), protein-losing states (nephrotic syndrome, protein-losing enteropathy), and malignancy (myeloma, chronic lymphocytic leukemia, thymoma). Genetic testing via targeted panels or whole-exome sequencing is increasingly employed.

Treatment

Lifelong IgG replacement therapy with individualized trough targets of 500 to 800 mg/dL or higher based on infection frequency forms the foundation of treatment. Prophylactic azithromycin may be used for bronchiectasis prevention. Autoimmune cytopenias are treated with rituximab, corticosteroids, or mycophenolate, with splenectomy avoided if possible due to the increased infection risk in an already immunocompromised host. GLILD is treated with rituximab combined with azathioprine (the combination with the most supporting evidence) or mycophenolate, with monitoring through pulmonary function tests and serial CT imaging. Cancer surveillance with clinical monitoring is important, with consideration of CT surveillance for lymphoma in high-risk subsets.

Targeted therapies are available for specific monogenic forms. CTLA-4 haploinsufficiency and LRBA deficiency respond to abatacept. Activated PI3K delta syndrome (APDS) is treated with leniolisib, the first targeted PI3K delta inhibitor approved for a primary immunodeficiency (FDA-approved 2023). STAT3 gain-of-function mutations may respond to JAK inhibitors such as ruxolitinib or tofacitinib.

<image>A clinical phenotyping diagram of CVID showing the four major clinical clusters. Central hub labeled "CVID" with four radiating categories: (1) Infection-only (~70%): icons for sinusitis, pneumonia, bronchiectasis, with chest CT showing bronchiectasis. (2) Autoimmune (~25-30%): icons for ITP (platelet with antibody), AIHA (red cell with antibody), autoimmune thyroiditis. (3) Polyclonal lymphoproliferation (~10-20%): chest CT showing GLILD (ground-glass opacities, nodules, mediastinal LAD), splenomegaly, granulomatous hepatitis. (4) Malignancy (5-8x risk): lymphoma cell, gastric cancer. Overlapping regions between categories showing patients with multiple phenotypes. Sidebar listing key immunologic markers: switched memory B cells <2% (poor prognosis marker), reduced plasmablasts, poor vaccine responses, and associated monogenic defects for each phenotype (CTLA4/LRBA with autoimmune, NFKB1/NFKB2 with infection).</image>

Selective IgA Deficiency

Selective IgA deficiency is the most common primary immunodeficiency, with a prevalence of approximately 1 in 500 in Caucasian populations (much rarer in Asian populations at approximately 1 in 18,000). It is defined as a serum IgA level below 7 mg/dL with normal IgG and IgM in a patient older than 4 years. The majority (85 to 90 percent) of individuals with selective IgA deficiency are asymptomatic. The symptomatic minority may experience recurrent sinopulmonary infections, gastrointestinal infections (particularly Giardia), celiac disease (with a 10- to 15-fold increased risk), and autoimmune diseases. A classically described though rare complication is the risk of anaphylaxis to blood products containing IgA, attributed to anti-IgA antibodies (IgG or IgE class); washed cellular products or IgA-deficient donor products should be used if transfusion is necessary. Approximately 5 to 7 percent of patients with selective IgA deficiency progress to CVID over time, and periodic monitoring of immunoglobulin levels is advisable. Selective IgA deficiency and CVID share genetic susceptibility loci in the MHC region and in TACI.

IgG Subclass Deficiency

IgG subclass deficiency is defined as deficiency of one or more IgG subclasses with normal total IgG levels. IgG2 subclass deficiency is the most clinically significant, as IgG2 antibodies are critical for responses to polysaccharide antigens from encapsulated organisms. The clinical significance of IgG subclass deficiency is debated and is considered meaningful only when associated with recurrent infections and demonstrably impaired specific antibody responses. IgG subclass deficiency may coexist with IgA deficiency. Immunoglobulin replacement therapy is reserved for patients with recurrent infections despite vaccination and antibiotic prophylaxis.

Specific Antibody Deficiency (SAD)

Specific antibody deficiency is characterized by normal quantitative immunoglobulin levels (IgG, IgA, and IgM are all within normal ranges) but impaired functional antibody responses to polysaccharide vaccines (Pneumovax 23), while responses to conjugate vaccines (Prevnar) remain intact. The assessment requires measuring titers to at least 12 to 14 serotypes, with an inadequate response defined as fewer than 70 percent of serotypes reaching protective levels (at least 1.3 mcg/mL). SAD predominantly affects children, and many outgrow the condition by age 6 to 7 years as the immune system matures. Treatment options include conjugate pneumococcal vaccination, prophylactic antibiotics, and immunoglobulin replacement in severe cases. Specific antibody deficiency may represent a precursor to CVID in some patients, warranting longitudinal monitoring.

Transient Hypogammaglobulinemia of Infancy (THI)

Transient hypogammaglobulinemia of infancy represents a prolonged physiologic nadir of IgG that extends beyond the expected 6 to 12 months of age. As maternal IgG wanes by 6 to 9 months, the infant's own IgG production may be slow to reach adequate levels. Most cases resolve by 2 to 4 years of age. THI is a diagnosis of exclusion and must be distinguished from XLA and early CVID. B cells are present and functional (unlike XLA), and specific antibody responses are usually normal. Most affected infants do not require immunoglobulin replacement, with antibiotic treatment of infections as needed.

Good Syndrome (Thymoma with Immunodeficiency)

Good syndrome is the association of thymoma with immunodeficiency, typically presenting in adults over 40 years of age. The immunologic defect involves combined T and B cell dysfunction, with hypogammaglobulinemia and reduced B cell numbers. Patients are susceptible to encapsulated bacteria, viruses (particularly CMV), fungi, and Pneumocystis jirovecii. Autoimmune manifestations are common, including pure red cell aplasia, myasthenia gravis, and graft-versus-host disease-like reactions. Treatment involves thymectomy (which may not reverse the immunodeficiency), lifelong immunoglobulin replacement, and antimicrobial prophylaxis.

Key Clinical Pearls

• XLA presents in early infancy with absent B cells, absent immunoglobulins, and absent tonsils; confirm with BTK protein expression or genetic testing
• CVID is heterogeneous: the autoimmune phenotype (especially autoimmune cytopenias) may precede the diagnosis of immunodeficiency by years
• Switched memory B cells <2% in CVID predicts autoimmunity, granulomatous disease, and splenomegaly
• Specific antibody response testing (pre/post Pneumovax) is essential: normal quantitative IgG does NOT exclude humoral immunodeficiency (SAD)
• CTLA-4 haploinsufficiency and LRBA deficiency are treatable causes of CVID-like disease (abatacept)
• Leniolisib (PI3Kdelta inhibitor) is the first targeted therapy approved for a PID (APDS/PI3Kdelta GOF)
• Selective IgA deficiency may evolve to CVID; monitor immunoglobulins periodically
• Live vaccines are contraindicated in XLA and severe CVID; killed vaccines should still be given (even with impaired response) in less severe humoral deficiencies

References

1. Bonilla FA, et al. International consensus document (ICON): common variable immunodeficiency disorders. J Allergy Clin Immunol Pract. 2016;4(1):38-59.
2. Conley ME, et al. Primary B cell immunodeficiencies: comparisons and contrasts. Annu Rev Immunol. 2009;27:199-227.
3. Warnatz K, et al. Severe deficiency of switched memory B cells in subgroups of patients with common variable immunodeficiency. Blood. 2002;99(5):1544-1551.
4. Rao VK, et al. Effective "activated PI3Kdelta syndrome"-targeted therapy with the PI3Kdelta inhibitor leniolisib. Blood. 2017;130(21):2307-2316.
5. Chapel H, et al. Common variable immunodeficiency disorders: division into distinct clinical phenotypes. Blood. 2008;112(2):277-286.

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