What blood tests reveal about your immune system

These markers show whether your immune system is balanced—or if dysfunction is quietly raising your risk of infection, autoimmune disease, and metabolic problems.

WRITTEN BY

Jessica Sebor

Updated: 10/07/2025|12 min read

Your immune system---comprising over a trillion cells, tissues, and organs, including white blood cells, mucus, bone marrow, and skin---detects, destroys, and protects you from threats like viruses and bacteria, and heals the body after infection or injury.

A healthy immune system is also essential for metabolic health. For example, autoimmune disorders can cause chronic inflammation, increasing the risk of developing cardiovascular disease, cancers, Type 2 diabetes, and more. Conversely, metabolic disorders can disrupt immunity. Take obesity, which increases the production of pro-inflammatory white blood cells called M1 macrophages. This creates a cycle of poor health where M1 macrophages disrupt insulin signaling pathways, leading to insulin resistance.

Understanding your immune response strength can help you take action to ensure it's neither under- nor overactive. The best way to get this insight is through blood testing. Checking the levels of specific immune biomarkers gives you a clear picture of where you stand and any risks that might be emerging.

Here's what you need to know about immune function, blood tests to check your immunity, and interpreting the results.

This information does not constitute medical advice. This is compiled expert opinion for educational purposes. Your doctor knows you best. Talk to your doctor for specific questions about your personal lab results.

Why does immune function matter for health?

The immune system helps keep us safe through a coordinated effort of organs, proteins, and specialized cells, some of which circulate in the blood. There are two branches: the innate and the adaptive immune system.

The innate immune system offers first-line protection. It's made up of:

• Our skin, which forms a physical barrier to keep invaders out of our bodies
• Mucous membranes lining the lungs, stomach, nose, and mouth, which secrete mucus to trap pathogens
• Phagocytes, natural killer cells, cytokines, and antimicrobial peptides---a team of cells, proteins, and amino acids that target pathogenic  invaders that slip past the skin and mucous membranes

If germs persist against these defenses, the adaptive immune system takes charge. T cells (a type of white blood cell) destroy infected cells and call upon other immune cells to help, while B cells (another white blood cell) create antibodies that attach to pathogens so they can't infect healthy cells. The adaptive immune system remembers each invader it encounters, so the next time it senses that foreign substance, it recognizes and attacks it sooner, leading to a milder illness or none at all.

Balance is key to immunity. A deficient immune system makes us more susceptible to disease and infection. This can happen for several reasons, from immunodeficiencies present at birth (called primary immunodeficiency) to acquired diseases like HIV/AIDs to temporary acquired deficiency after a virus. Lifestyle factors, including excessive drinking, smoking, eating processed foods, lack of sleep, and high stress, can also weaken the immune system.

On the other hand, an overactive immune system can respond to harmless substances or attack healthy tissues. Autoimmune diseases (or autoimmune disorders) are the classic example. These include Type 1 diabetes, lupus, multiple sclerosis, and rheumatoid arthritis. The immune system can also become overactive due to allergic reactions (like hay fever), chronic infections (like Lyme disease and Epstein-Barr virus), and post-viral syndromes (like long COVID and shingles).

The immune system doesn't work in isolation. It has a powerful influence on our metabolism, nervous system, cardiovascular system, musculoskeletal system, and more, so a dysregulated immune system can drive other negative health effects. For example, dysfunctional immunity can lead to inflammation, which in turn causes issues with insulin sensitivity, contributing to metabolic syndrome---a known risk factor for Type 2 diabetes, cardiovascular disease, and stroke. In short: If our immune system isn't working as it should, it puts nearly every aspect of our health in peril.

When should you consider a blood test for immune function

Doctors typically check white blood cell count (WBC) as part of an annual physical, which often includes a blood differential test that measures the levels of different types of white blood cells. A deeper look at immune function (like an immunoglobulins test) usually isn't ordered unless there's evidence of an underlying issue.

You may want to ask your doctor for a test or order one through an independent lab if:

• Your white blood count is off. If a complete blood count (CBC) indicates that your white blood cell count is high or low (normal range is 4,000 to 10,800 cells per microliter), your immune system may not be functioning properly.
• You suffer from frequent or repeated infections. Persistent colds, diarrhea, sinus and ear infections, and yeast infections may indicate an immunodeficiency.
• You're experiencing symptoms of autoimmune disorders, such as chronic fatigue, joint pain, swollen glands, and digestive issues.
• You have a family history of immune disorders, since some of these diseases are genetic.
• You're interested in your overall health. An immune system blood test helps establish a baseline for future comparisons and may reveal early signs of imbalance, allowing you to be proactive in your health.

What white blood cell tests tell you about immune function

White blood cells (leukocytes) make up 1 percent of your total blood volume, yet they defend your entire body. A white blood cell count (WBC) with differential is typically the starting point for immune testing, providing the first indication that the balance between different types of white blood cells may be off.

It's best to wait to test if you've recently experienced an illness or injury, which can temporarily elevate white blood cell levels for up to eight weeks.

Always interpret your results with the help of a medical professional. Normal ranges and units of measurement may vary depending on your lab, and complicating factors (like medication) can affect your levels.

Keep in mind that research shows it can be normal for some people to have lab results that fall slightly outside the standard range, especially if those levels remain stable over time. For example, research indicates that some people are predisposed to have a genetically lower white blood cell count with no other health issues. And those with African, West Indian, and Middle Eastern descent are predisposed to what's called "benign ethnic neutropenia," meaning they naturally have lower levels of the white blood cell type neutrophils.

When you're ready to dive in, here's what those results can tell you.

White blood count (WBC)

White blood count measures the total number of pathogen-fighting cells in your blood. While your WBC is a key indicator of how your immune system is functioning overall, it's not the whole story. More white blood cells aren't necessarily better, as both high and low levels can be cause for concern. Plus, the balance between different types of white blood cells and other aspects of the immune system also matters.

Normal range: 4,500 to 10,800 cells per microliter (cells/μL)

A low WBC (leukopenia) can be caused by a weakened immune system, bone marrow problems, certain viral and bacterial infections, or the effects of medications like chemotherapy. Having low levels of white blood cells to protect your body increases your risk of infection from invaders like bacteria, viruses, and fungi.

A high WBC (leukocytosis) may indicate infection, inflammation, tissue damage, pregnancy, acute emotional or physical stress, or, in rare cases, blood cancers like leukemia. Cigarette smoking and certain drugs can also raise your WBC. Leukocytosis can increase the risk of mortality, and very high levels (hyperleukocytosis, defined as a WBC over 100,000 cells per microliter) can lead to blood clots.  

Neutrophils 

The most abundant type of white blood cell, neutrophils are first responders. They can attack invaders by engulfing them and releasing enzymes that break them down. They also help regulate inflammation by releasing signaling proteins that attract other immune cells, coordinate the intensity of the response, and help transition the body from fighting infection to healing.

Normal range: 40 to 73 percent of absolute WBC (1,500 to 8,000 cells/μL)

Low neutrophil counts (neutropenia) may result from chemotherapy, viral infections, autoimmune conditions, and nutrient deficiencies, including vitamin B12, folate, and copper. Neutropenia can increase the risk of serious infections, especially when levels drop severely (less than 500 cells/μL).

Elevated neutrophils (neutrophilia) often indicate an infection. However, high counts can also be triggered by physical or emotional stress, acute injury, inflammation, certain types of leukemia, intense exercise, or smoking.

Monocytes

Produced in our bone marrow, monocytes are large white blood cells constantly on alert for signs of foreign invaders. When they sense a threat, they swarm the site, enter the tissues, and transform into macrophages (which surround and kill germs) and dendritic cells (which coordinate other cells to fight the infection).

Normal range: 4 to 10 percent of WBC (200 to 1,000 cells/μL)

Low monocytes (monocytopenia) can occur due to bone marrow suppression, burn injuries, or blood infections. Monocytopenia can increase your risk of infection.

High readings (monocytosis) are often a sign of high inflammation due to viruses (like mononucleosis) or conditions like cardiovascular disease, blood disorders, or autoimmune diseases like lupus or rheumatoid arthritis. Chronically high monocyte levels can impair the healing process and exacerbate the progression of existing disease.

Lymphocytes 

Lymphocytes are white blood cells that primarily defend against bacteria and viruses. They include B cells, which produce antibodies, and T cells, which help destroy infected or abnormal cells. As part of the adaptive immune system, lymphocytes "store data" on past infections to help the body mount a faster, stronger response if the same threat returns. The other lymphocytes, natural killer cells (NKC), destroy cells that were once healthy but have become harmful, such as cancer.

Normal range: 19 to 48 percent of WBC (1,000 to 4,000 cells/μL)

Lymphopenia (low levels of lymphocytes) may also occur due to mild infections, malnutrition, a side effect of medication, or, in rarer cases, an autoimmune disease, Hodgkin's disease, or HIV/AIDS. Like with other types of white blood cells, lower levels increase your risk of getting sick.

Lymphocytosis (high levels) usually means the body is fighting a viral infection (like the flu or cold), but it can be a symptom of a more serious issue, such as hypothyroidism, HIV/AIDS, or blood cancer.

Eosinophils

Eosinophils are a type of white blood cell that defends the body against parasites, responds to allergens, and fights certain kinds of bacteria.

Normal range: 0 to 7 percent of WBC (0 to 500 cells/μL)

Low eosinophil count (eosinophenia) is not usually a cause for concern on its own, as it may be brought on by acute infections or stress. However, this can also indicate more serious issues like Cushing's syndrome or sepsis, which require further testing to evaluate and confirm.

Allergic reactions (like hay fever or asthma), parasitic infections, and autoimmune disorders can all cause high eosinophil count (eosinophilia). Sometimes this condition can occur in a specific body part, like the lungs or bladder, leading to localized issues.

Basophils 

Basophils are the least common type of white blood cell. They play a role in allergic reactions and inflammation. In response to an allergen or injury, they release substances, resulting in three actions:

• The release of histamine causes blood vessels to dilate and become more permeable, increasing blood flow so that immune cells can more quickly reach the site of infection or injury. 
• When histamine sets off an allergic reaction, you experience as itching, watery eyes, a runny nose, rashes, sneezing, swelling, and more.
• Heparin, an anticoagulant, is released. This prevents blood clots where damage has occurred, allowing immune cells in the blood to reach the infected site faster.

Normal range: Up to 2 percent of WBC (0 to 200 cells/μL)

Allergic responses and infections can cause low basophil counts (basopenia). This is also a symptom of hyperthyroidism (overactive thyroid).

Too many basophils (basophilia) can also occur due to infections or allergic reactions, as well as due to chronic conditions, including inflammatory bowel disease (IBD), hypothyroidism (underactive thyroid), rheumatoid arthritis, and leukemia.

Neutrophil to lymphocyte ratio (NLR)

The neutrophil to lymphocyte ratio is increasingly used as a marker of systemic inflammation and immune system balance, and is associated with an increase in chronic disease risk and mortality. It's calculated by dividing the neutrophil count (innate immune system) by the lymphocyte count (adaptive immune system).

Normal range: 1--2

A low NLR suggests lower levels of inflammation in the body, which is generally a good thing. However, a very low ratio (0.7 or less) may indicate neutropenia, which raises your risk of infection.

A high NLR may indicate chronic inflammation, which may be caused by everything from stress or infection to cardiovascular disease, diabetes, or some cancers. A high NLR has also been studied as a predictor of poor outcomes in hospitalized patients, and can increase with age and is elevated in people with obesity, as these factors are linked to chronic, low-level inflammation.

Immature granulocytes

Granulocytes (including neutrophils, eosinophils, and basophils) develop in the bone marrow and shouldn't be released into the bloodstream until they're mature. So a high level of immature granulocytes may be cause for concern.

Normal ranges:

• Ages 10+: less than 0.90 percent
• Ages <10: less than 0.30 percent

Values higher than 1 percent can indicate a "left shift." This can indicate inflammation if your body is in the early stages of fighting an infection, or it might mean a bone marrow disorder. Healthy pregnant women and newborns can also see high levels.

Nucleated red blood cells (NRBC)

Nucleated red blood cells are immature red blood cells that still contain a nucleus. Similar to immature granulocytes, these cells are typically found only in the bone marrow. This biomarker is sometimes connected with a worse prognosis and risk of mortality for critically ill hospitalized patients.

Normal range: 0-0.03 percent (0 per 100 WBC)

In adults, elevated NRBC may indicate serious conditions such as anemia, leukemia, hemorrhage, bone marrow disorders, severe hypoxia, sepsis,  or significant inflammation or physical stress such as trauma or critical illness.

Other tests to assess immune function

White blood cells are a critical component of the immune system, but they're only one part of it. For a more complete picture of your defensive line, you might consider other blood tests.

Immunoglobulin tests measure the levels of antibodies your body uses to fight off infections, and C-reactive protein (CRP) reflects the level of inflammation in your body. Unlike white blood cell tests---which count the cells involved in immune responses---these tests focus on the signals and substances your immune system produces.

Immunoglobulin test

Immunoglobulins (Ig or antibodies) are Y-shaped proteins produced by B cells called plasma cells. When immunoglobulins encounter an antigen (virus, harmful bacterium, fungus, or toxin), they bind to it. This neutralizes the invader or tags it for other immune cells to come and destroy it.

Each type of immunoglobulin has a distinct location in the body and a unique role in immunity. An immunoglobulin blood test---also known as a quantitative nephelometry test---typically measures the three main types of antibodies in your bloodstream. While each antibody measurement has slightly different indications, overall low levels leave you more prone to disease and infection, while high levels are often linked to an increased risk of mortality.

1. IgG (immunoglobulin G) is the most abundant antibody in the body, providing long-term protection by remembering past infections or vaccinations. That way, when the body encounters a pathogen that it recognizes, the immune response is faster and stronger, leading to a milder illness or infection.

• Normal range: 767 to 1,590 milligrams per deciliter (mg/dL)
• Low IgG levels may indicate malnutrition, immune disorders like agammaglobulinemia (a rare inherited disorder), blood cancers (such as leukemia or multiple myeloma), cancer treatments (chemotherapy), or preeclampsia in pregnancy.
• High levels may indicate the early stages of a disease. It may also signal chronic infection or inflammation, autoimmune disorders like rheumatoid arthritis, liver disease, or, in rare cases, multiple myeloma.

2. IgM (immunoglobulin M) is the first responder to infection. It helps to quickly fight off germs and activate other parts of the immune system.

• Normal range: 37 to 286 mg/dL
• Low IgM may be caused by rare immune disorders like agammaglobulinemia or cancers such as leukemia and multiple myeloma, autoimmune diseases, gastrointestinal diseases, and immunosuppressive treatments for autoimmune disorders.
• IgM levels rise in response to infection. Chronic elevation can indicate autoimmune diseases such as rheumatoid arthritis, or blood cancers, including a type of lymphoma and multiple myeloma.

3. IgA (immunoglobulin A) is found in mucous membranes, where it helps defend areas like the respiratory and digestive tracts from infection.

• Normal range: 61 to 356 mg/dL
• Low IgA levels may result from hereditary IgA deficiency, rare immune disorders like agammaglobulinemia, multiple myeloma, or gastrointestinal diseases that cause protein loss.
• High levels may be due to chronic infections (especially in the gut), inflammatory bowel diseases like Crohn's or ulcerative colitis, or multiple myeloma.

C-reactive protein (CRP)

C-reactive protein is made mostly in the liver and rises rapidly---up to 1,000 fold---when there's injury or infection, helping the immune system respond. The CRP test is a way to measure inflammation in the body.

• Normal range: Less than 0.3 milligrams per deciliter (mg/dL)
• Minor elevation: 0.3 to 1.0 mg/dL. It can occur thanks to lifestyle factors, including cigarette smoking and being sedentary, and conditions like depression, diabetes, obesity, and gingivitis.
• Moderate elevation: 1.0 to 10.0 mg/dL. Often indicates more chronic inflammatory issues, including Crohn's disease, ulcerative colitis, lung disease, and autoimmune disorders like lupus and rheumatoid arthritis. 
• Marked elevation: above 10.0 mg/dL. Suggests severe inflammation caused by serious injury or infection.
• Acute elevation: above 50.0 mg/dL. Indicates acute bacterial infection like sepsis.

What's next if your immune markers are out of range?

Low or high immune markers don't always indicate something serious. Your body may simply be fighting something off. But if you haven't recently been sick and aren't healing from an injury or surgery, you could have chronic inflammation or an immune system imbalance that can indicate anything from an autoimmune disorder to chronic stress to blood cancer.

To be certain, let your doctor know about your current health status and lifestyle. They may order follow-up tests to pinpoint the cause, check for underlying issues, and help guide the next steps.

Get your immune function levels tested

Levels members can test dozens of markers, including a those for immune function, through Levels Labs, and then use the Levels app to help improve levels by logging meals and setting exercise goals. Click here to learn more about Levels.