Are you worried about the health of your immune system? Here are some of the useful tests your doctor can order based on your needs to make sure that your immune system is working optimally.
1) White Blood Cells (Leukocytes)
When we are talking about immunity and immune system health, there’s no better place to start than white blood cells (WBC) or leukocytes. These cells protect us against infections and diseases by fighting bacteria, viruses, and other foreign invaders. That is why, when you are checking the health of your immune system, the first step should be to get a complete blood count (CBC) with a WBC differential (or alternatively just a WBC differential).
CBC gives you information about your blood cells: red blood cells (RBCs), white blood cells (WBCs), and platelets. It’s a common test that is normally a part of a routine health exam. Doctors use it to help detect a wide range of health issues, including anemia, infections, clotting problems, immune system disorders, and blood cancers [1, 2].
When it comes to white blood cells, having low WBC levels, for example, can mean that your body doesn’t have adequate resources to fight against infections properly. High WBC, on the other hand, may mean that you’re currently fighting an infection or that there’s inflammation in the body.
White Blood Cell Differential
If your white blood cells are decreased or increased, it is important to check which particular cells are the cause. WBC differential means that, apart from giving a total white blood cells count, the test will also include the counts and/or percentages of the 5 major types of white blood cells:
- Neutrophils — these first-responders at infection and inflammation sites help fight infection by ingesting microbes and releasing enzymes that kill them [3, 4]
- Lymphocytes — these are cells that produce antibodies against microbes, kill cancer or virus-infected cells, and help direct the immune response [5, 6, 7, 8]
- Monocytes — cells that kill microbes, ingest foreign particles, remove dead cells, and boost the immune response [9, 10, 11].
- Eosinophils — cells that fight parasite infections and are involved in the allergic response 
- Basophils — cells involved in inflammatory responses [6, 13, 14].
Having a normal percentage and level of each different type of WBC is important for your health . Remember, to always work with your doctor to find out what’s causing abnormal test results. Your doctor will interpret your results in conjunction with your symptoms, your medical history, and other tests, and may order additional tests if necessary.
2) C-Reactive Protein (hs-CRP)
C-reactive protein or CRP is a useful test because it can tell whether your immune system is engaged in fighting an infection. Better yet, it can tell you if you are experiencing chronic inflammation which is closely tied to your metabolic health .
The liver makes CRP in response to trauma, inflammation, and infection [16, 17]. CRP’s job is to then bind damaged tissue or invading microbes and tag them so the immune system (white blood cells) can recognize them and clear them away [18, 19].
Once the cause of the disturbance has been taken care of, CRP levels usually drop back to normal. However, when there’s chronic low-grade inflammation in the body, CRP levels remain slightly increased .
There are two types of CRP tests. A regular CRP test is ordered for people with symptoms of a serious bacterial infection or inflammatory diseases. It measures CRP in the range from 10 to 1,000 mg/L [21, 22].
Bacterial infections can massively increase CRP (40 – 200 mg/L or more), while viral infections and mild inflammation usually trigger a more subtle rise in CRP (10 – 40 mg/L) [23, 24].
However, if you’re interested in checking for low-grade chronic inflammation or for an indication of your heart disease risk, you should do a hs-CRP test. hs-CRP test measures CRP in the range from 0.5 to 10 mg/L. The sensitivity of the hs-CRP test allows the detection of slightly elevated CRP levels that would otherwise go unnoticed with a regular CRP test .
hs-CRP and Metabolic Health
It is now generally accepted that hs-CRP levels can help predict the future risk of heart (cardiovascular) disease even in apparently healthy people [23, 25, 26]. CRP levels are associated with heart disease risk, as follows [25, 27, 28]:
- Low risk: hs-CRP level under 1.0 mg/L
- Average risk: between 1.0 and 3.0 mg/L
- High risk: above 3.0 mg/L
- Very high risk: 5-10 mg/L
- Above 10 mg/L – clinically significant inflammatory states
Lack of sleep, stress, smoking, unhealthy diets, nutrient deficiency have all been associated with higher CRP levels [29, 30, 31, 32, 20, 33, 34, 35, 36].
Higher CRP levels have also been found in metabolic syndrome, obesity, obstructive sleep apnea, and diabetes [37, 38, 39, 40, 41, 42, 43, 44, 45, 46].
Alternatives to testing hs-CRP/CRP
There are other tests that doctors can use when they want to check if there is inflammation or an ongoing infection in the body, such as the ESR.
Erythrocyte sedimentation rate, or ESR, is often called the “sickness index”. It used to be tested more often, before the hs-CRP tests became available. Doctors still use it to track the progress of specific diseases [47, 48, 49, 50].
3) Globulins (Gamma Gap)
Globulin levels, also sometimes called the gamma gap, are a measure of proteins other than albumin in the blood, including alpha, beta, and gamma globulins. They can help your doctor diagnose a viral or bacterial infection, inflammatory disorder, or an autoimmune disease. This is because antibodies are gamma globulins and their increased production can signal these diseases .
However, it’s not just elevated levels that serve as a signal for disease. Low levels can also indicate there’s a health issue. If your results are out of the normal range, work with your doctor to discover what underlying condition might be the cause and to develop an appropriate plan to improve your health.
Specific Antibody Tests
If needed, your doctor will order more in-depth antibody tests. There are, for example, tests that look at the levels of the four major types of antibodies:
- IgM — Their levels increase during the initial phase of a new infection, and they provide general but short-term protection against infections. IgM levels eventually decline as the body starts producing more IgG antibodies [52, 53, 54].
- IgG — These are the most common antibodies in the blood. They are made in the second wave of the immune response and are crucial for a successful defense against viruses and bacteria [55, 53, 54].
- IgA — Rather than the blood, these antibodies are found in the mucus or the gut, lungs, and urogenital tract. They are the first line of defense against harmful microbes we breathe in or ingest [53, 54].
- IgE — These antibodies protect against parasites but are also involved in allergic reactions [53, 54].
There is another type of antibody — IgD. However, scientists still don’t know much about it or what it does [53, 54]. That’s why it’s not tested except for research.
Finally, there are also tests that look at specific antibodies against certain diseases (e.g. VCA IgG and IgM tests for Epstein-Barr virus, hepatitis B and C antibodies, or antithyroid antibodies). Doctors use these to confirm or rule out a potential disease.
4) Vitamin D (25-hydroxyvitamin D)
Vitamin D, or the “sunshine vitamin”, is best known for its role in building and keeping bones strong and healthy. It is less known, however, that this vitamin also plays an important role in the immune system and has effects on both the innate and the adaptive immune response [56, 57].
In fact, studies have shown that vitamin D deficiency increases the risk of developing certain bacterial and viral infections. Scientists also think that vitamin D may be important for keeping excessive inflammation at bay [58, 59, 60, 61].
This is important in the light of large population studies, which suggest that around 42% of U.S. adults are deficient in this vitamin [62, 63].
Vitamin D is produced by the skin upon exposure to sunlight. It can also be obtained in the diet, or through vitamin supplements .
People who are at risk of vitamin D deficiency are those:
- Living at higher latitudes, for example Canada or northern parts of the US [65, 66, 67].
- Living in regions where there are large seasonal changes in sun exposure. In winter, for example, when people are less likely to go out .
- Living in areas with high levels of air pollution, which blocks out sunlight .
- Having darker skin [70, 71].
- Over-using sunscreen .
- Keeping the skin covered up (in colder climates or certain cultures) [73, 74].
If you are worried about your immune health, making sure you are not vitamin D deficient would be a step in the right direction. This is especially true if you are at a higher risk of vitamin D deficiency. To check your levels, ask your doctor to order a 25-hydroxyvitamin D test.
Much like other essential nutrients, zinc plays many important roles in the body, including its role in the immune system .
Zinc is essential for the normal development and function of many immune cells. Because of the critical role it plays, even a mild deficiency can impair immune function and increase the risk of bacterial and viral infections. Scientists found that zinc deficiency, which is common in older people, reduces immune defenses and increases the risk of walking pneumonia and other infections [76, 77, 78, 79, 80, 81].
According to some estimates, zinc deficiency contributes to about 16% of lower respiratory infections .
You may be at risk for zinc deficiency if you [83, 84, 85]:
- have a digestive disorder
- are pregnant or lactating
- are an alcoholic
- are vegetarian
- have diabetes or prediabetes
- have sickle cell disease
If you suspect that you have a zinc deficiency (symptoms include: diarrhea, frequent infections, hair loss, eye and skin conditions, delayed wound healing, impotence, and loss of appetite), your doctor can check your levels by ordering a blood test .
Studies suggest that in the elderly and in people who have immunodeficiency (e.g., sickle cell disease, HIV infection), zinc supplementation can restore immune function and the resistance to infections [87, 88, 89, 90]. However, it’s important to dose with zinc carefully because excessive levels can have the exact opposite effects by suppressing immunity [91, 92, 93].
If you are deficient in this nutrient, work with your doctor to devise a supplement plan that best works for you and your health condition.
Iron is another nutrient that plays an essential role in the immune system and resistance to infections [94, 95]. It is necessary for producing [96, 97]:
- white blood cells
- reactive oxygen species required for killing microbes
Iron deficiency interferes with immune function and can skew the immune response away from Th1 and towards Th2, which lowers the body’s ability to fight off viral and bacterial infections. For example, iron deficiency anemia has been linked to more frequent respiratory infections in children [98, 99, 100].
Women and children are more likely to experience iron deficiency . Symptoms include: fatigue, headaches, pale skin, anxiety, and shortness of breath [102, 103, 104, 105, 106, 107, 108]. If you have any of these, ask your doctor to check your iron.
Your doctor can order the following blood tests to check your iron levels:
- Iron + TIBC (Total iron binding capacity) — these are measured together to give an indication of iron deficiency.
- Transferrin saturation (% saturation) – it’s derived from your iron and TIBC levels.
- Ferritin — usually a good measure of the body’s total iron stores, but can be unreliable if there’s an infection or inflammation in the body .
There are studies that suggest iron supplementation may be beneficial to prevent some types of respiratory infections [110, 111].
However, while iron deficiency hampers the immune response and recovery in some infections, it seems to help with other infections, such as malaria or tuberculosis. That’s because some microbes actually need iron to thrive [112, 113].
Furthermore, excessive iron levels in the body can be toxic, which is why it’s best to work with your doctor on building the best supplement regimen that’s tailored to your needs.
Magnesium is the 4th most abundant mineral in the body. It is required for more than 300 different bodily processes, including the activation of vitamin D. Because of this, a deficiency can wreak havoc on your health, including your immune system [114, 115, 116, 117].
Some reports suggest that magnesium deficiency may increase susceptibility to viral infections . This is especially important in the light of how prevalent magnesium deficiency is.
One study estimates that 75% of the U.S. population doesn’t get enough magnesium in their diet! In addition to a poor diet, certain groups of people are at a higher risk of deficiency due to other causes, including malabsorptive disease, diabetes, alcoholism, or old age [118, 119, 120].
Finally, emotional stress and stressful activities can increase the loss of magnesium from the body .
Symptoms linked with low magnesium include: a loss of appetite, fatigue, nausea, insomnia, irritability, headaches, and muscle weakness [122, 121].
If you are experiencing these symptoms, you’re under a lot of stress and/or have a condition that puts you at risk, it’s a good idea to ask your doctor for a magnesium blood test. If your levels are on the low side, up your magnesium intake through magnesium-rich foods or supplements.
Folate, also known as vitamin B9, is water-soluble essential vitamins. It is needed for making new cells and this includes immune cells [123, 124, 125].
Scientists found that people who have folate deficiency have an impaired immune response (mainly cell-mediated immunity) and are more susceptible to infections [124, 126].
A large scale population study found that less than 1% of adults in the US were folate deficient. However, up to 20% of women had folate insufficiency. Folate insufficiency means that there isn’t enough folate for the body to function optimally . Folate deficiency may be higher in countries without folic acid fortification.
Could you be suffering from folate deficiency or insufficiency? Symptoms of low folate include: weakness, fatigue, brain fog, irritability, headaches, pounding or racing heartbeat, and shortness of breath .
Now, bear in mind that folate deficiency rarely occurs in isolation. Rather, it is usually found with other nutrient deficiencies, such as vitamin B12 deficiency, and it’s associated with :
- A poor diet (folate is found mainly in fresh fruits and vegetables)
- Malabsorptive disorders (e.g. celiac disease)
If you have any of these disorders, a poor diet, and/or folate deficiency symptoms, it may be a good idea to check your folate levels. Discuss getting a folate test with your doctor.
Are your folate levels on the low side? Remember, the best way to increase folate is to up your intake of fresh fruits and vegetables. Cooking can eliminate as much as 90% of the folate content of cooked foods .
If you do need to take folate supplements, choose 5-MTHF over folic acid (the synthetic version of vitamin B9) because the latter is not absorbed as well [130, 131].
9) Vitamin C
Vitamin C is an essential nutrient and antioxidant that contributes to immune defense by supporting both the innate and the adaptive immune response. It can enhance the killing of microbes and can decrease tissue damage due to an overactive immune response .
That’s why it’s not surprising that vitamin C deficiency can impair immunity and increase susceptibility to infections .
Nowadays people tend to think that vitamin C deficiency is a thing of the past. However, according to the National Health and Nutrition Examination Survey, about 5 -17% of adults in the US are vitamin C deficient! Smokers, in particular, are at a high risk of vitamin C deficiency [133, 134].
Furthermore, infections can deplete vitamin C levels due to enhanced vitamin C requirements by the immune system .
There is evidence that supplementation with vitamin C may both prevent and treat respiratory and systemic infections [132, 135, 136].
If you’re a smoker, overweight, or you’re suffering from chronic diseases, it may be a good idea to test your vitamin C levels and adjust your diet accordingly. Alternatively, your doctor may recommend supplements.
In general, prevention of infection requires lower levels of vitamin C intake (i.e., 100–200 mg/day), while during an infection the body requires significantly higher (gram) doses .
What About Other Nutrients?
There are several other nutrients that can also affect the immune system. Namely, vitamins A, E, B2, B6, B12, and selenium .
While deficiencies in these nutrients may be more prevalent in developing countries (e.g. vitamin A deficiency), they are all pretty rare in the US . For example, less than 1% of the US population is deficient for vitamin A or vitamin E .
However, bear in mind that the risk of nutrient deficiencies generally increases with age and in certain health conditions .
For example, vitamin B12 deficiency is one of those deficiencies that is practically nonexistent in the younger population, but becomes more common in old age. Around 4% of the elderly in the US have it .
Selenium deficiency, on the other hand, tends to be regional, as the food content depends on the concentration of selenium in the soil .
The best thing you can do for your immune system is to eat a balanced, healthy diet, in order to prevent nutritional deficiencies.
If you’d like to know more about your immune health, you can ask your doctor to test some of the following, based on your conditions, symptoms, and lifestyle factors:
- White blood cells
- C-reactive protein
- Vitamin D
- Vitamin C
This list is a good place to start. Your doctor will run additional tests if necessary.
- White blood cells
- C-reactive protein
- Vitamin D
- Iron / ferritin
- Vitamin C
- Vitamin A
- Vitamin E
- Vitamin B2
- Vitamin B6
- Vitamin B12