Low IgG and cancer are connected in two directions, and for most people who notice the link, the cancer or its treatment came first. Certain blood cancers and many modern cancer drugs quietly reduce the body’s supply of infection-fighting antibodies. The main practical concern is preventing infections, which is something doctors can measure and treat. People born with weak antibody production face the reverse problem, a higher chance of a few specific cancers.
How Blood Cancers Lower IgG
IgG is the most plentiful antibody in your blood. Your report may list low levels as “hypogammaglobulinemia,” which simply means low antibody levels. IgG is made by plasma cells, which grow out of B cells, a type of white blood cell. Cancers of B cells or plasma cells therefore sit right inside the antibody-making system.
In myeloma, cancerous plasma cells multiply and crowd out the healthy ones that make useful antibodies.1PubMed Central. IgG replacement in multiple myeloma Over 9 in 10 people newly diagnosed with myeloma have reduced levels of working antibodies.2PubMed Central. IgG replacement in multiple myeloma It also shows up in 25–40% of people with MGUS, a harmless-looking precursor, and in 75% of people whose myeloma is in remission.3PubMed Central. IgG replacement in multiple myeloma Low antibodies can persist even when the cancer is under control.
A normal-looking total protein can hide the problem. In myeloma, the cancer cells flood the blood with one identical antibody-like protein, often called a paraprotein or M-protein, that does nothing useful against germs. Because it counts toward the total, the overall protein number can look normal or even high while the antibodies that fight infection are low. That’s why “my protein was fine” doesn’t settle the question, and why a separate measurement matters.
In chronic lymphocytic leukemia (CLL), about 1 in 4 people already have low IgG at diagnosis, before any treatment.4PubMed Central. Hypogammaglobulinemia in Newly Diagnosed Chronic Lymphocytic Leukemia: Natural History, Clinical Correlates and Outcomes When IgA and IgM are counted too, more than half of newly diagnosed people are low in at least one antibody type.5PubMed Central. Prevalence and the impact of hypogammaglobulinemia in newly diagnosed chronic lymphocytic lymphoma patients The cancerous B cells don’t make normal antibodies, and they disrupt the healthy ones.
On your blood report, IgG, IgA and IgM are usually listed separately, each with its own normal range. The adult IgG range is roughly 700–1,600 mg/dL, though labs vary and some report in g/L (divide mg/dL by 100). Ask which antibody is low, because IgG matters most for infection decisions. Low IgG often isn’t mentioned at diagnosis, and oncologists may check it only after infections start.
Cancer Treatments That Lower IgG
Rituximab removes B cells, the source of new antibody-making cells. In children treated with rituximab, about half developed low antibody levels afterward.6PubMed Central. Persistent hypogammaglobulinemia after rituximab therapy in pediatric patients, prevalence and clinical outcomes In another group of children, the share with low IgG rose from about 29% before treatment to 43% after, and IgM lows rose from 20% to 62%.7PubMed. Rituximab-induced hypogammaglobulinemia and infection risk in pediatric patients In about half of affected children the low levels lasted more than 6 months, and some stayed low beyond 5 years.8PubMed Central. Persistent hypogammaglobulinemia after rituximab therapy in pediatric patients, prevalence and clinical outcomes Starting with low IgG and IgM made a long-lasting drop more likely.9PubMed Central. Persistent hypogammaglobulinemia after rituximab therapy in pediatric patients, prevalence and clinical outcomes
The drop can show up months after the drug. Rituximab clears B cells quickly, but antibodies already circulating in your blood last for weeks, so IgG falls gradually rather than overnight. A lab drawn early may look fine and be falsely reassuring. Rechecking at set intervals is what catches the drop before infections do.
BTK inhibitors such as ibrutinib, widely used for CLL, block signals B cells need.10PubMed Central. Exploring the depths of hypogammaglobulinemia in lymphoid malignancies: pathophysiology, clinical implications, management options, and future directions IgG tends to fall gradually over the months and years of treatment.11PubMed Central. The Immunologic Paradox of BTK Inhibitors in Chronic Lymphocytic Leukemia: Selectivity, Hypogammaglobulinemia, and Infection Risk Sinus, chest, skin and urinary infections become more likely.12PubMed Central. Exploring the depths of hypogammaglobulinemia in lymphoid malignancies: pathophysiology, clinical implications, management options, and future directions
Newer immune therapies act the same way. CAR T-cell therapy, which targets B cells or plasma cells, can also cause low IgG.13PubMed Central. Hypogammaglobulinemia due to CAR T-cell therapy Bispecific antibodies for myeloma wipe out normal plasma cells along with cancerous ones.14PubMed Central. Immunoglobulin prophylaxis should be initiated after bispecific antibody therapy in multiple myeloma, regardless of IgG levels Because of that, some experts recommend antibody infusions after bispecific therapy regardless of the IgG number.15PubMed Central. Immunoglobulin prophylaxis should be initiated after bispecific antibody therapy in multiple myeloma, regardless of IgG levels Low IgG from the cancer plus a B-cell drug can stack, and each later round of treatment can push levels lower.
Before starting one of these drugs, ask whether it is known to lower antibodies. Ask for an IgG level before the first dose and a schedule for rechecking. Ask whether vaccines should be given before treatment starts, since responses are better then.
What Low IgG Means for Infections
The infections that follow low antibodies are mostly bacterial infections of the airways and lungs.16PubMed. Serum immunoglobulins and risk of infection: how low can you go? In myeloma, the chest was the most common site of serious infection.17PubMed Central. Immunological factors and risk of infection in plateau phase myeloma Disease activity matters too: serious infections were about four times more common while myeloma was active than when it was stable.18PubMed Central. Immunological factors and risk of infection in plateau phase myeloma
Very low levels held for a long time, such as IgG below 100 mg/dL, carry a risk of repeated and sometimes life-threatening infections.19PubMed. Serum immunoglobulins and risk of infection: how low can you go? But risk starts rising well above that. In lymphoma patients on bendamustine-rituximab, an IgG low point under 800 mg/dL made infection about five times more likely.20Journal of Clinical and Experimental Hematopathology. Serum IgG and lymphocyte counts are useful for the early detection of infection in patients receiving bendamustine-rituximab therapy
The number isn’t everything. In myeloma, people often make weak antibodies against pneumonia bacteria and tetanus even after vaccination.21PubMed Central. Immunological factors and risk of infection in plateau phase myeloma A poor response to the pneumonia vaccine was linked to a higher risk of blood infection.22PubMed Central. Immunological factors and risk of infection in plateau phase myeloma Doctors can measure antibodies before and a few weeks after a pneumonia or tetanus shot, and a weak rise suggests they aren’t working well. Live vaccines are usually avoided when IgG is low.
A worrying pattern looks like sinus infections or bronchitis that keep returning within weeks, several antibiotic courses a year, or infections that take longer than usual to clear. A pneumonia or a hospital stay for infection counts as severe. Keeping a log helps: note each infection’s date, type and whether antibiotics were needed. Doctors use the count and severity to decide on treatment, and a log is especially useful when several doctors prescribe antibiotics.
Infections are not only about antibodies. Cancer and its treatments can also lower neutrophils, the white blood cells that fight bacteria, and steroids can weaken other parts of the immune system.
Treating Low IgG With Antibody Infusions
Guidelines recommend replacement when IgG is below 400 mg/dL (4 g/L) and you’ve had more than 3 infections or one severe infection.23PubMed Central. Immunoglobulin substitution in patients with secondary antibody deficiency in chronic lymphocytic leukemia and multiple myeloma: a representative analysis of guideline adherence and infections It may be considered with IgG under 400 mg/dL and one to three milder infections.24PubMed Central. Immunoglobulin substitution in patients with secondary antibody deficiency in chronic lymphocytic leukemia and multiple myeloma: a representative analysis of guideline adherence and infections25PubMed Central. Immunoglobulin prophylaxis should be initiated after bispecific antibody therapy in multiple myeloma, regardless of IgG levels
It works well. In people with CLL, infections of moderate severity or worse dropped by about two-thirds after starting infusions into a vein.26Cancer Research Communications. Intravenous Immunoglobulin Reduces Infections for Patients with Chronic Lymphocytic Leukemia: A Single-Center Retrospective Analysis In a trial in myeloma, none of the people receiving infusions under the skin developed blood infection or bacterial pneumonia, versus dozens in the untreated group.27PubMed Central. IgG replacement in multiple myeloma People with CLL needed about a third as many antibiotic prescriptions after starting under-the-skin infusions.28Blood. The Efficacy and Cost Effectiveness of Subcutaneous Immunoglobulin (SCIG) Replacement in Patients with Immune Deficiency Secondary to Chronic Lymphocytic Leukemia
Antibody replacement comes in two forms. Infusions into a vein (IVIG) are usually given every 3–4 weeks in a clinic. Infusions under the skin (SCIG) are usually weekly and can be done at home.29PubMed Central. IgG replacement in multiple myeloma SCIG keeps IgG steadier and causes fewer whole-body side effects.30Frontiers in Hematology. Immunoglobulin replacement therapy in hematologic malignancy–associated secondary immunodeficiency: roles of SCIG and IVIG Its main downside is local skin redness or swelling.31PubMed Central. IgG replacement in multiple myeloma
An IVIG day takes a few hours in an infusion chair, and headache, tiredness or chills are common the next day. SCIG uses small needles in the belly or thigh, and people are often taught in one or two visits. Doctors recheck the trough IgG, the level just before the next dose, to adjust it.
Daily preventive antibiotics are another option.32Blood Advances. Economic evaluation: immunoglobulin vs prophylactic antibiotics in hypogammaglobulinemia and hematological malignancies In one cost comparison, antibiotics were cheaper and no worse for quality of life than antibody infusions.33Blood Advances. Economic evaluation: immunoglobulin vs prophylactic antibiotics in hypogammaglobulinemia and hematological malignancies Antibiotics suit people with fewer, milder infections, while infusions suit people with serious or repeated infections despite antibiotics. Insurance approval often requires an IgG number and an infection history, and the choice can change as your disease and treatment change.
Infusions don’t replace every kind of protection. Donor antibodies cover many common germs, but they are borrowed protection that wears off, which is why doses repeat. They don’t restore your own ability to make antibodies, and they don’t fix other weaknesses such as low neutrophils. You can still get infections on replacement, and a breakthrough infection calls for a conversation with your team, not necessarily a sign the treatment has failed.
When Low IgG Comes First and Cancer Follows
CVID, common variable immunodeficiency, is a lifelong condition in which the body makes too few antibodies. People with CVID get stomach cancer about 16 times, skin cancer other than melanoma about 13 times, and non-Hodgkin lymphoma about 12 times as often as expected.34PubMed Central. Common Variable Immunodeficiency-Associated Cancers: The Role of Clinical Phenotypes, Immunological and Genetic Factors In absolute terms, about 4% of people with CVID develop lymphoma and about 1.5% stomach cancer.35PubMed. Malignancy in common variable immunodeficiency: a systematic review and meta-analysis A risk 16 times higher is multiplied against a small starting number, so most people with CVID never develop these cancers.
The cancers aren’t mainly from weak immune “patrol.” They come from the same faulty genes or from chronic infection and inflammation.36PubMed. Intrinsic and extrinsic causes of malignancies in patients with primary immunodeficiency disorders People with IgA deficiency have 7 to 10 times the usual rate of stomach cancer.37PubMed Central. Primary Immunodeficiency and Cancer Predisposition Revisited: Embedding Two Closely Related Concepts Into an Integrative Conceptual Framework They struggle to clear H. pylori, a stomach bacterium.38PubMed Central. Primary Immunodeficiency and Cancer Predisposition Revisited: Embedding Two Closely Related Concepts Into an Integrative Conceptual Framework The risk appears to fall when the bacteria are wiped out.39PubMed Central. Primary Immunodeficiency and Cancer Predisposition Revisited: Embedding Two Closely Related Concepts Into an Integrative Conceptual Framework
With CVID, ask to be tested for H. pylori and treated if positive. Report swollen lymph nodes, night sweats or weight loss promptly. Regular skin checks make sense given skin cancer risk, and your immunologist can say whether stomach scopes are advised for you.
The reverse question comes up too. Low IgG found in adulthood is sometimes the first sign of CLL, myeloma or a thymus tumor, so doctors often check blood counts and protein tests to rule these out. Most adults with low IgG don’t turn out to have cancer.
Thymoma and Low IgG
A tumor of the thymus, a gland in the chest, can come with very low antibodies, called Good’s syndrome.40PubMed Central. Clinical and laboratory features of seventy‐eight UK patients with Good’s syndrome (thymoma and hypogammaglobulinaemia) Almost everyone with it first comes to a doctor because of infections, and nearly half develop damaged airways.41PubMed Central. Clinical and laboratory features of seventy‐eight UK patients with Good’s syndrome (thymoma and hypogammaglobulinaemia) About a quarter have autoimmune conditions such as myasthenia gravis or thyroid disease.42PubMed Central. Clinical and laboratory features of seventy‐eight UK patients with Good’s syndrome (thymoma and hypogammaglobulinaemia)
Removing the tumor usually doesn’t restore antibody levels, and most people need lifelong antibody infusions. Because this tumor can be the cause, chest scans are sometimes done in adults with unexplained low IgG.
Solid Tumors and Immunotherapy
Across many studies, people with solid cancers had IgG levels similar to people without cancer.43PubMed. Association of Serum Immunoglobulin Levels with Solid Cancer: A Systematic Review and Meta-analysis In stomach cancer, though, lower IgG went with more advanced disease and shorter survival.44Yonago Acta Medica. Decreased Serum Concentration of Total IgG Is Related to Tumor Progression in Gastric Cancer Patients
In melanoma, higher IgG before starting checkpoint immunotherapy went with longer control of the cancer.45PubMed. Immunoglobulin G and Subclasses as Potential Biomarkers in Metastatic Melanoma Patients Starting Checkpoint Inhibitor Treatment Long-term checkpoint drugs may affect antibodies, but this isn’t proven and routine testing isn’t advised.46PubMed Central. Checkpoint Blockade and Acquired Humoral Immune Dysregulation: Emerging Evidence for Antibody Deficiency During Long-Term PD-1/PD-L1 Inhibition Testing makes sense if you have repeated or unusual infections on these drugs.47PubMed Central. Checkpoint Blockade and Acquired Humoral Immune Dysregulation: Emerging Evidence for Antibody Deficiency During Long-Term PD-1/PD-L1 Inhibition
For someone with a solid tumor, routine IgG checks usually aren’t needed. Ask for one if infections keep returning. A prior rituximab course or a blood cancer history changes this.
Will IgG Recover After Treatment
In children finishing chemotherapy, up to 89% had low antibody levels.48PubMed. Immune recovery in children with malignancy after cessation of chemotherapy 81% still had some immune problem 9 to 12 months after stopping, and younger children recovered more slowly.49PubMed. Immune recovery in children with malignancy after cessation of chemotherapy After rituximab, low IgG lasted more than 5 years in some children.50PubMed Central. Persistent hypogammaglobulinemia after rituximab therapy in pediatric patients, prevalence and clinical outcomes Recovery is slow because B cells must be regenerated from the bone marrow and then mature into plasma cells, which takes many months.
Ask for repeat IgG checks every few months after treatment ends. Infusions can sometimes be stopped once levels recover on their own, and childhood vaccines may need repeating after chemotherapy.