Assessing immune function: an easy way to get started


If you would like an easy way to get started with assessing your immune function, this simple test might surprise you, because you may find you’ve already done it!

Has your doctor ever ordered a full blood count (FBC) when you went there for tests? Chances are you have at least one such test in your medical file. And it is precisely the most accessible way to start assessing your immune function, especially if it’s a FBC with differential.

What is a Full Blood Count with Differential and how is it useful in getting started with assessing your immune function?

Blood is, simply put, liquid with some dissolved chemicals, and lots of cells floating about. The FBC looks at these cells, of which white blood cells (WBC) are your circulating immune cells. Getting an idea as to what is the WBC number, as well as, ideally, the numbers of the different kinds of WBC (a.k.a. the differential), is the simplest way to get started with assessing your immune function.

The relevance of tests – even before we get started with assessing immune function

A cautionary word first: any lab test (except genetic tests) is just a snapshot at the moment of the sample collection. In the FBC case, it’s the moment of the blood draw. Some test results may change dramatically from one moment to the other due to different factors. Others naturally show marked differences throughout the day.

Ideally, serial assessments would provide a more accurate picture. This is because they would show a trend as opposed to one isolated instance. Assessing your immune function based on FBC is no exception.

We need to interpret tests in the context of the person’s health and accounting for the different circumstances.

What, where and how much?

Needless to say, looking for the right marker in the right place for the right reason will yield the most relevant conclusions.

For example, basophils are one of the immune cells populations identified on the FBC with differential. They participate, among other things, in allergic reactions. However, the number of basophils in the blood is very low, almost insignificant, and their counterparts that reside in tissues, called mast cells, will carry out most of the allergic process. Whilst basophil count may be elevated in allergic disease, it is not necessarily the main marker checked by clinicians (1).

Another type of WBC, eosinophils, is in higher numbers in blood and we can see elevations of this immune cells fraction in allergies and parasitic infections (2).

The other immune cells fractions routinely identified in blood are neutrophils, lymphocytes and monocytes, of which the former two represent the majority of WBC.

Neutrophils are innate immune cells. They carry chemical warfare and are equipped to engulf invaders or tissue debris, without any target specificity. If there is a bacterial infection where bacteria reside outside the host’s cells (i.e., a sinus, lung, digestive or urinary tract infection), then we would expect higher neutrophil count, thus higher immune activation associated with destruction of foreign or host cells (by the immune system or by the pathogen, respectively).

What about autoimmunity?

Assessing immune function is very important in cases of autoimmunity.

Interestingly, even neutrophil elevations outside the context of infection, may be a sign that the immune function is skewed towards destruction. This is because neutrophils will answer to the same inflammatory signals that nurture the destructive lymphocytes involved in autoimmunity (3). In such patients, with autoimmunity but no obvious infection, high neutrophils may be a sign of autoimmune flare and may warrant mitigating measures.

However, in cases of autoimmunity, when the immune system ends up attacking the body’s own structures, the immune response is an adaptive one.

It is possible to find low lymphocyte counts (4).

It is also possible to identify alarm signals that reveal the need for an immune system rebalancing intervention. For example, if the ratio between neutrophils and lymphocytes shifts from a normal neutrophil prevalence towards increased lymphocytes, then it may be a sign that an adaptive immune response becomes prevalent. In people with autoimmunity, the risk of developing further autoimmune diseases is greater. That is because of the tendency of their immune systems to learn about the self structures as if they were foreign (it identifies then attacks them). The lymphocytes proliferation may be a sign of an ongoing autoimmune process.

After you get started with assessing immune function: what do the findings mean

Is ‘within range’ the only good sign on FBC?

Matching expectations is an important part of interpreting lab tests, and this also applies to the FBC with differential.

All reports will be contain ‘reference ranges’, and results that fall within the range are deemed ‘normal’. If the person is apparently healthy, then we would expect a normal FBC. Would a WBC count within range be normal, though, if an obvious aggressive infection is present? Not at all. In that clinical context, one would expect immune activation and immune cells proliferation. If WBC populations are all within ranges, one would question the reason for that and would investigate immune function further.

High vs. low vs. normal

Any infection doesn’t necessarily mean high WBC counts, though. There is a timeline of deviations from normal. In some cases, these dynamic changes are not intuitive without further knowledge about how the immune system functions.

Let’s take the case of an infection with viruses or intracellular bacteria. In such cases we would expect a rise in the lymphocytes fraction of WBC. Lymphocytes are part of the adaptive immune system. It means that they learn specific details about their targets and can precisely identify them. Some lymphocytes can directly engage intruders, others create substances, called antibodies (yes, like the ones after vaccination), that fight against dangerous structures by specifically binding to them. Once a lymphocyte has learnt about its target and can precisely identify it, it multiplies rapidly and creates a clone army that will stand a good chance at eliminating the intruder. At that moment, the lymphocytes count on the FBC will be high.

However, the body has protective mechanisms that limit this cellular expansion. The clone population will naturally decrease. In some cases a few memory cells will remain, keeping the knowledge about that specific intruder in their genetic code and being able to proliferate again in a new clone army just as well trained as the initial one.

This reduction of the clone cells happens even if in some cases the infection is still present. The decrease in lymphocytes may have multiple mechanisms (5). That is the case when, although there is infection still present, WBC counts appear normal or the lymphocyte count is low. Again, interpreting lab results in context and on a timeline is essential and can inform the best interventions to help the patient.

Percentage vs. absolute count

A clarification is in order here. The FBC with differential report shows each of the WBC subsets both as percentage and absolute count. The latter is self explanatory – the numbers are the actual counts of each type of cell per volume of blood and deviations from ranges will reflect actual excesses or deficiencies.

The percentage of each population can fall above or beyond expected ranges even though actual numbers are still within normal. This is because the percentage reflects a ratio between different WBC subtypes. For example, the lymphocyte percentage may appear low, even if the lymphocyte count is normal, if the number of neutrophils increases and proportionally they predominate. Or, the lymphocyte percentage may appear high, despite normal lymphocyte count, if the neutrophils number is low, because neutrophils no longer are the dominant immune cell subtype in the blood. Or, as suggested above as an autoimmunity alarm signal, actual numbers of lymphocytes and neutrophils may be within range, but the proportion may change in favour of lymphocytes.

The final words of wisdom about assessing your immune function via FBC

A FBC with differential is an easy way to get started with assessing your immune function.

Obviously, there may be other clinical situations that affect the results of the FBC with differential, such as different forms of cancer, some treatments, or even congenital/inherited variants, alongside inflammation (immune activation) in the context of allergy, autoimmunity or infection.

However, this is not a lab tests interpretation course. Nor is it an exhaustive list of possible health clues we can derive from a FBC with differential. A healthcare practitioner should make sense of the information derived from different investigations. Nevertheless, the informed patient, aware of the value of such data, will make a better partner in their own health journey.


It is important to appreciate that the immune system is a reactive part of our bodies. Its adequate reaction in the clinical context is a good sign even when numbers fall outside reference ranges.

Of equal importance is understanding that, in order to recommend adequate and timely interventions, we may need to detect subtle changes that may not cause ‘abnormal’ results.

The right healthcare practitioner, knowledgeable and interested in your health and aware of the importance of immune balance, will be your ideal problem solver, and, perhaps more importantly, your facilitator of optimal health.

At The Allergy-Immunology Doctor, each patient is unique and we strive to help them towards their best possible health. If you would like to learn more about how we may be able to help, you may wish to book a free discovery call:


1. Feriel J, Depasse F, Geneviève F. How I investigate basophilia in daily practice. Int J Lab Hematol. 2020 Jun;42(3):237–45.

2. Kuang FL. Approach to Patients with Eosinophilia. Med Clin North Am. 2020 Jan;104(1):1–14.

3. Mills KHG. IL-17 and IL-17-producing cells in protection versus pathology. Nat Rev Immunol. 2023 Jan;23(1):38–54.

4. Sheu TT, Chiang BL. Lymphopenia, Lymphopenia-Induced Proliferation, and Autoimmunity. Int J Mol Sci. 2021 Apr 16;22(8):4152.

5. Guo Z, Zhang Z, Prajapati M, Li Y. Lymphopenia Caused by Virus Infections and the Mechanisms Beyond. Viruses. 2021 Sep 20;13(9):1876.