Causes of Low IgG Levels

Primary Immunodeficiency Causes

The American Academy of Allergy, Asthma, and Immunology recommends that X-linked agammaglobulinemia (Bruton disease) be considered in patients with absent or extremely low B cells and profoundly low IgG levels 1
Autosomal recessive agammaglobulinemia causes similar severe IgG deficiency with absent B cells, typically manifesting with recurrent bacterial infections of the upper and lower respiratory tract 1
Common Variable Immunodeficiency (CVID) should be suspected in patients over 4 years old with IgG below 450-500 mg/dL, low IgA levels, and impaired antibody responses, affecting approximately 1 in 30,000 persons 2
CVID presents with recurrent sinopulmonary infections from encapsulated bacteria like H. influenzae and S. pneumoniae, and requires exclusion of secondary causes including medications, protein loss, B-cell lymphomas, and bone marrow failure 2

Medications such as antiepileptic drugs, gold, penicillamine, hydroxychloroquine, and NSAIDs can cause secondary IgG subclass deficiency 5
Gastrointestinal protein loss, renal protein loss, and lymphatic protein loss can present with low serum total protein and/or albumin levels alongside low IgG 2
B-cell lymphomas can cause secondary hypogammaglobulinemia 2
HIV infection causes secondary immunodeficiency with variable IgG abnormalities 5
Post-hematopoietic stem cell transplantation (HSCT) and bone marrow failure can also lead to low IgG levels 5, 2

Normal IgG for a 4-year-old should be above 450-500 mg/dL, and IgG levels gradually increase with age, reaching adult-like levels by 59 months 4
Measuring specific antibody responses to protein and polysaccharide vaccines is essential, as IgG levels alone do not predict antibody production capacity 2
Do not diagnose IgG subclass deficiency based solely on laboratory values without clinical correlation, and normal total IgG does not exclude subclass deficiency 5
Approximately 2.5% of healthy individuals naturally have IgG subclass levels below 2 standard deviations of the mean, and some patients with IgG subclass deficiency may evolve into CVID over time 5