Systemic lupus erythematosus, especially when it involves the kidneys, is the autoimmune disease most consistently linked to low IgG levels. But lupus is far from the only one. Vasculitis, rheumatoid arthritis, juvenile idiopathic arthritis, dermatomyositis, and autoimmune cytopenias have all been associated with drops in IgG, sometimes from the disease itself and sometimes from the drugs used to treat it. Untangling which factor is responsible turns out to be one of the trickiest problems in clinical immunology.
Lupus Nephritis Is the Clearest Example
Lupus, particularly when it attacks the kidneys, has the strongest documented relationship with low IgG. The connection is surprisingly straightforward: when lupus damages the kidney’s filtering apparatus, large proteins that normally stay in the blood start spilling into the urine. IgG is one of those proteins. In a study of patients with active lupus nephritis, roughly a quarter had reduced IgG levels, and the drop tracked closely with how much protein was leaking through the kidneys.
The more severe the protein loss, the worse the IgG depletion. Among patients spilling more than 8 grams of protein per day, about 42 percent had low IgG. In those losing more than 14 grams daily, three out of four were IgG-deficient.1BMJ Open. Correlation of hypogammaglobulinaemia with proteinuria, and the relationship between hypogammaglobulinaemia and infection in active lupus nephritis A separate study confirmed this pattern, finding that reduced IgG occurred in 26 percent of active lupus nephritis patients overall, driven primarily by how much protein the kidneys were letting through.2PubMed. Serum immunoglobulin G level in patients with lupus nephritis and the effect of treatment with corticosteroids and mycophenolate mofetil
That said, kidney leakage doesn’t explain everything. Some lupus patients with low IgG aren’t losing much protein in their urine at all. Researchers have found that urinary loss can account for the IgG drop in some patients, but in many others, the antibody levels are low for reasons that don’t line up neatly with what’s coming out in the urine.3PubMed. Urinary loss of immunoglobulin G anti-F(ab)2 and anti-DNA antibody in systemic lupus erythematosus nephritis That points to additional mechanisms at work beyond simple protein spilling.
Other Autoimmune Diseases Linked to Low IgG
Lupus gets the most attention, but several other autoimmune conditions show up in this picture. A pediatric review identified systemic lupus erythematosus, juvenile idiopathic arthritis, vasculitis, and dermatomyositis or polymyositis as conditions associated with secondary low IgG.4PubMed Central. Secondary Hypogammaglobulinemia: Diagnosis and Management of a Pediatric Condition of Clinical Importance Among these, vasculitis patients appear to face a higher likelihood of developing infections from their low immunoglobulin levels compared to patients with other rheumatic diseases.
Rheumatoid arthritis has an interesting and somewhat counterintuitive relationship with IgG. While the disease typically involves overactive immune responses, a subset of patients actually present with low immunoglobulin levels across the board, including IgG below what would be considered normal. An older but notable study found that rheumatoid arthritis patients with these low immunoglobulin levels paradoxically had milder disease, with better functional capacity and fewer joint erosions compared to patients with higher antibody levels.5PubMed Central. A benign form of rheumatoid arthritis, associated with low IgG, IgM and IgA and C3-proactivator concentrations in the serum This suggests that in some people, the immune dysregulation driving the autoimmune disease and the low antibody levels may share a common root rather than one causing the other.
Granulomatosis with polyangiitis, a type of vasculitis that affects small blood vessels, is another condition where low IgG comes up frequently. Patients with this disease often receive rituximab as part of their treatment, making it difficult to separate how much IgG loss comes from the disease versus the drug. Baseline IgG levels in these patients tend to run lower than average even before treatment begins, hinting that the disease itself plays a role.6BMC Musculoskeletal Disorders. Low immunoglobulin levels increase the risk of severe hypogammaglobulinemia in granulomatosis with polyangiitis patients receiving rituximab
The Biological Mechanisms Behind the Drop
Low IgG in autoimmune disease isn’t caused by just one thing. Several distinct processes can pull antibody levels down, and they sometimes stack on top of each other in the same patient.
- Urinary loss: As described with lupus nephritis, damaged kidneys let IgG escape into the urine. The worse the kidney damage, the more IgG is lost.
- Hypercatabolism: The body breaks down IgG faster than normal. Research going back decades has shown that patients with connective tissue diseases catabolize IgG at an accelerated rate, and this is a defect in the patient’s system rather than a problem with the antibodies themselves.
- Gut protein loss: Some autoimmune or inflammatory conditions cause protein-losing enteropathy, where proteins including IgG leak through the intestinal wall. In one cohort of children with early-onset protein-losing enteropathy, 72 percent had low IgG.
- Impaired production: Chronic immune activation and the medications used to manage it can both suppress the B cells responsible for making new IgG.
The hypercatabolism angle deserves a closer look because it’s often overlooked. In a classic study, researchers demonstrated that patients with several different connective tissue diseases all broke down normal IgG at an accelerated rate. The IgG they tested wasn’t defective; it was normal protein from healthy donors. Yet it disappeared from the patients’ blood faster than expected.7PubMed Central. Hypercatabolism of normal IgG; an unexplained immunoglobulin abnormality in the connective tissue diseases Later animal research pointed to a specific recycling receptor called FcRn as the likely culprit. This receptor normally rescues IgG from being broken down, extending its lifespan in the blood. In lupus-prone mice, disease-related impairment of FcRn function shortened IgG’s half-life to less than a third of its normal value.8PubMed. Hypercatabolism of IgG in mice with lupus-like syndrome
Protein-losing enteropathy is a less common but dramatic cause of IgG loss. When the gut lining becomes leaky enough to let large proteins through, immunoglobulins drain out along with albumin and other blood proteins. The 72 percent figure from a Chinese pediatric cohort illustrates how severe this can be.9PubMed. Phenotype and Genotype of a Cohort of Chinese Children with Early-Onset Protein-Losing Enteropathy Conditions like inflammatory bowel disease and intestinal lymphangiectasia, which can overlap with autoimmune processes, are among the causes.
Rituximab and Treatment-Induced IgG Loss
Here’s where things get genuinely complicated: many of the drugs used to treat autoimmune diseases also lower IgG. Rituximab, a medication that depletes B cells to calm overactive immune responses, is the biggest offender. It’s used widely across rheumatology and hematology for conditions like lupus, vasculitis, rheumatoid arthritis, and autoimmune cytopenias. And it reliably drives IgG levels down.
The numbers are striking. In a 10-year real-world study of autoimmune disease patients treated with rituximab, about 63 percent developed low IgG, with an obvious decline showing up as early as three months after starting the drug.10PubMed. Hypogammaglobulinemia and Infection Events in Patients with Autoimmune Diseases Treated with Rituximab: 10 Years Real-Life Experience Rituximab works by wiping out CD20-positive B cells, which normally take six to twelve months to recover, though in some patients the depletion lasts considerably longer.11PubMed Central. Rituximab Use and Hypogammaglobulinemia
For some patients, the IgG drop is temporary. For others, it becomes a lasting problem. A study in pediatric patients found that about 55 percent developed low IgG after rituximab, and of those who could be followed long enough, about half had persistent low levels lasting more than six months. Nine patients remained IgG-deficient for over five years. Patients who had lower IgG or IgM levels before starting rituximab were more likely to end up with persistent deficiency.12Clinical Immunology Communications. Persistent hypogammaglobulinemia after rituximab therapy in pediatric patients, prevalence and clinical outcomes
Tracking IgG over time in rituximab-treated patients tells a nuanced story. In one cohort of 177 patients with multisystem autoimmune disease, 13 percent had IgG below the threshold before their first dose. That proportion crept up to 17 percent at two years and stayed around 14 percent at five years, suggesting a relatively stable rate of deficiency overall. But over the full follow-up, about a third of patients dipped below that threshold for at least three consecutive months, and 4 percent hit severely low levels. Higher steroid doses during treatment and cumulative rituximab exposure both appeared to push levels lower.13BMC Musculoskeletal Disorders. The effect of rituximab therapy on immunoglobulin levels in patients with multisystem autoimmune disease
Other treatments can also contribute. Therapeutic plasma exchange, sometimes used for myasthenia gravis and other antibody-mediated conditions, physically removes IgG from the blood. A single course of plasma exchange can reduce immunoglobulin levels by roughly 60 to 70 percent.14PubMed Central. Effect of therapeutic plasma exchange on immunoglobulins in myasthenia gravis Standard plasma exchange removes IgG more effectively than double-filtration techniques.15PubMed. Ability to remove immunoglobulins and anti-ganglioside antibodies by plasma exchange, double-filtration plasmapheresis and immunoadsorption The levels typically recover between sessions, but repeated exchanges can keep IgG chronically suppressed. Corticosteroids, mycophenolate, and other immunosuppressants also contribute to impaired antibody production, though their individual effects are harder to isolate since they’re rarely used alone.
When Low IgG Signals a Primary Immune Deficiency
One of the most important clinical puzzles is figuring out whether a patient’s low IgG is being caused by their autoimmune disease, by their medications, or by an underlying primary immune deficiency that was there all along. This matters because the management is different. A patient whose IgG dropped from rituximab might recover when the drug is stopped or switched. A patient with an inborn immune deficiency won’t.
Common variable immunodeficiency, the most frequently diagnosed primary antibody deficiency in adults, has a complicated relationship with autoimmune disease. Autoimmune blood problems like immune thrombocytopenia and autoimmune hemolytic anemia can actually be the first sign of this condition, appearing before a patient ever gets a serious infection.16Hematology Am Soc Hematol Educ Program. Common variable immunodeficiency: autoimmune cytopenias and advances in molecular diagnosis In other words, a patient who presents with what looks like a straightforward autoimmune condition may have an underlying immune deficiency driving both the autoimmunity and the low antibodies.
This overlap is especially important in children. In a study of kids who received rituximab for autoimmune cytopenias, about a third developed persistent low IgG lasting more than a year after treatment. But when researchers dug deeper, over half of those children eventually turned out to have an underlying primary immune deficiency.17PubMed Central. Rituximab Unveils Hypogammaglobulinemia and Immunodeficiency in Children with Autoimmune Cytopenia Rituximab, in a sense, unmasked a vulnerability that might otherwise have gone undetected for years. Children who developed persistent low IgG were younger at the time of treatment, had slower B-cell recovery, and were more likely to have autoimmune hemolytic anemia or Evans syndrome.
Autoimmunity in pediatric patients more broadly can be a red flag for an underlying inborn error of immunity. A review of secondary low IgG in children noted that autoimmunity in this age group may be representative of an underlying primary immunodeficiency, making careful follow-up essential even when the low IgG seems to have an obvious explanation.4PubMed Central. Secondary Hypogammaglobulinemia: Diagnosis and Management of a Pediatric Condition of Clinical Importance
What Happens When IgG Stays Low
Low IgG isn’t just a number on a lab report. IgG is the immune system’s workhorse antibody, responsible for recognizing and neutralizing bacteria, viruses, and other pathogens you’ve encountered before or been vaccinated against. When levels drop far enough, the practical consequence is infections, particularly respiratory and sinus infections that keep coming back.
Whether low IgG actually leads to more infections depends on how low the levels go and what else is happening with the patient’s immune system. Mild dips may cause no problems at all. In a study of patients with mastocytosis who happened to have low immunoglobulin levels, the low levels weren’t typically associated with chronic or recurrent infections.18PubMed Central. Assessment of low immunoglobulin levels and clinical manifestations in patients with mastocytosis But in patients with autoimmune diseases who are already on immunosuppressive medications, even a moderate IgG drop can tip the balance toward frequent or severe infections. Vasculitis patients appear to be at particularly high risk compared to those with other rheumatic conditions.
When IgG falls very low and infections become a recurring problem, immunoglobulin replacement therapy becomes an option. In the 177-patient rituximab-treated cohort mentioned earlier, about 3 percent eventually needed intravenous immunoglobulin infusions, all of whom had IgG levels below 5 g/L combined with recurrent infections.13BMC Musculoskeletal Disorders. The effect of rituximab therapy on immunoglobulin levels in patients with multisystem autoimmune disease That’s a relatively small percentage, which suggests that most patients with treatment-related IgG dips don’t end up needing replacement, but the minority who do can be quite sick without it.
FcRn Inhibitors and the Intentional Lowering of IgG
In a twist that might seem paradoxical, some newer autoimmune treatments are specifically designed to lower IgG. These drugs, called FcRn inhibitors, target the same recycling receptor whose impairment contributes to IgG loss in lupus. By blocking FcRn, they prevent the receptor from rescuing IgG molecules, causing them to be broken down faster and clearing pathogenic autoantibodies from the blood.19PubMed Central. FcRn inhibitors: a novel option for the treatment of myasthenia gravis
The strategy is being explored most actively for generalized myasthenia gravis, a condition where autoantibodies attack the junction between nerves and muscles. Because FcRn inhibitors reduce all IgG subtypes, they can potentially work regardless of which specific autoantibody subclass is causing the problem.20Journal of the Neurological Sciences. What Autoimmune Diseases Cause Low IgG? The approach flips the usual concern on its head: instead of worrying about low IgG as a side effect, clinicians are harnessing the body’s own IgG clearance pathway as a therapeutic tool. The challenge, of course, is calibrating the dose so that harmful autoantibodies are cleared without suppressing protective IgG so much that infections become a problem. That balance is still being refined as these drugs move through clinical development for myasthenia gravis and potentially other antibody-driven conditions.
Why Monitoring IgG Matters During Long-Term Autoimmune Treatment
If you’re living with an autoimmune disease and taking immunosuppressive medications, periodic IgG monitoring is something worth discussing with your doctor, especially if you’ve been on rituximab for multiple cycles or are receiving a combination of immune-suppressing drugs. The risk factors for developing clinically meaningful low IgG are reasonably consistent across studies: lower baseline immunoglobulin levels before treatment, higher cumulative drug exposure, concurrent steroid use, and younger age at treatment onset.
Not every dip in IgG requires action. Many patients tolerate moderately low levels without increased infections. The practical threshold for concern is usually when levels fall well below normal and infections start occurring more often or more severely than expected. Clinicians typically look at the whole picture: IgG level, infection frequency, the specific autoimmune disease being treated, and what alternative medications might be available if the current regimen is driving antibody levels dangerously low. For a small number of patients, immunoglobulin replacement infusions become necessary to bridge the gap while the underlying treatment continues or is adjusted.