Does absent or very low IgG increase the risk of recurrent infections?

Immunoglobulin G (IgG) is the most abundant antibody class in human serum, serving as a cornerstone of the adaptive immune system. Research confirms that severe deficiency or absence of IgG—whether primary or secondary—substantially impairs the body’s ability to neutralize and clear pathogens, leading to a high clinical burden of recurrent infections.

Clinical evidence and infection risk

The correlation between low IgG levels and infection frequency is well-established in clinical literature.

• Infection frequency: Individuals with severe IgG deficiency (typically defined as levels >2 standard deviations below the age-adjusted mean) experience significantly higher rates of sinopulmonary infections, including pneumonia, sinusitis, and bronchitis.
• Specific metrics: Longitudinal studies in older adults show that low IgG levels are an independent risk factor for pneumonia, with a hazard ratio (HR) of 1.13 for each unit decrease in specific immunoglobulin subclasses. In patients with secondary hypogammaglobulinemia, studies indicate that immunoglobulin replacement therapy (IgRT) can reduce infection frequency by an average of 2.5 episodes per year.
• Pathogen susceptibility: Patients are particularly vulnerable to encapsulated bacteria such as Streptococcus pneumoniae and Haemophilus influenzae, which require robust antibody responses for clearance.

Mechanistic explanations

The increased susceptibility to infection is driven by the failure of two primary immunological processes:

• Neutralization failure: IgG normally binds to viral spike proteins, bacterial adhesions, or microbial toxins. This binding physically blocks the pathogen from interacting with host cell receptors, effectively "neutralizing" the threat before it can enter cells. In the absence of IgG, pathogens can disseminate and infect tissues with minimal resistance.
• Impaired opsonization: IgG serves as a molecular "tag" (opsonin) that facilitates phagocytosis. The Fc portion of the IgG molecule binds to Fc-gamma receptors (FcγR) on the surface of macrophages and neutrophils. This interaction triggers the engulfment and destruction of the pathogen.
• Complement activation: IgG is a potent activator of the classical complement pathway. Low IgG levels prevent the deposition of C3b on pathogen surfaces, which further reduces the efficiency of phagocytosis and the formation of the membrane attack complex (MAC).

Considerations for older adults

In a 71-year-old female, the clinical impact of low IgG is often exacerbated by immunosenescence and secondary factors.

• Secondary causes: In older populations, low IgG is frequently secondary to hematological conditions (such as Chronic Lymphocytic Leukemia) or the use of B-cell depleting therapies (like rituximab).
• Reduced resilience: Aging is associated with a decline in "naive" B-cell diversity and increased T-cell exhaustion. Consequently, an older patient with low IgG has fewer compensatory immune mechanisms to manage infections compared to a younger individual, leading to higher morbidity and mortality from respiratory diseases.

Bottom line

Absent or very low IgG levels directly impair the dual mechanisms of pathogen neutralization and opsonization, resulting in a documented increase in recurrent, severe infections. For older patients, maintaining adequate IgG levels is critical for reducing pneumonia risk and improving overall survival.