Immune system: how your resistance works and what blood shows

Your immune system is not a switch that sits on or off. It works in layers, around the clock, and blood shows you only part of it. Adults catch roughly two to four colds a year, and that is normal (PMID 12517470).

Here is what strikes me about most pages on this topic: nearly all of them end at a jar of supplements. Almost none explain what a blood test can and cannot say here.

That is a missed opportunity. No blood value grades your resistance, and that honest answer helps you more than another list of tips.

Below you will find which layers your immune system is built from, which values a doctor sometimes looks at, and where the limits of blood testing sit.

What is your immune system made of?

Your immune system has three layers that flow into each other. First the physical barrier: your skin, your mucous membranes and your stomach acid. Behind that sits innate immunity, which reacts fast and without a target. Last comes adaptive immunity, which remembers specific pathogens (PMID 20176265).

That first layer is almost always skipped. Your skin is the largest barrier you have. A break in it, or a dried out mucous membrane, lets through more than most people assume.

The second layer is made of cells that attack anything that is not yours. Phagocytes literally eat pathogens. This layer needs no memory to start working.

The third layer is the slow, clever one. B cells make antibodies against one specific intruder. T cells clear out cells that are already infected.

That last layer remembers what it met before. It is why measles normally happens once.

It is also why colds catch you again every year. Dozens of different cold viruses exist, and immunity to one gives no protection against the next (PMID 12517470).

What is the difference between innate and adaptive immunity?

Innate immunity is fast and untargeted. It responds within minutes to hours to anything foreign. Adaptive immunity is slow and specific: antibodies take days to appear, but afterwards your body often recognises that one pathogen for years (PMID 20176265).

Your results show both layers in different numbers. Granulocytes belong mainly to the fast layer. Lymphocytes belong to the learning layer.

That is why a full blood count does not report one number for white cells, but a breakdown. That breakdown is the leukocyte differential.

If you want the detail on that fast layer, the article on granulocytes goes deeper. For the learning layer, the piece on low lymphocytes is the place to start.

Which blood values say something about your immune system?

No single value measures your resistance as a whole. Some values do say something about parts of it: the number and type of white blood cells, the antibody classes you produce, and whether inflammation is active somewhere. Which value is useful depends entirely on the question underneath it.

The table below sets out the situations that come up most often. Read it as a thinking aid, not as an order form.

Pay attention to that last column. It matters as much as the third one, and it is exactly what nearly every page on this subject leaves out.

How do you know whether you have a good immune system?

No test answers that. This is not evasion, it is where the science stands. Your immune system is an interplay of cells, proteins and barriers that responds differently in every situation, and a single snapshot in blood does not capture that in one number.

What you do have is your behaviour over time. How often do you get ill, how long does it last, and do you recover normally?

Those three questions say more in practice than any isolated figure.

Dutch health information service Thuisarts describes a cold as an infection that usually clears on its own within one to two weeks. If your pattern differs from that structurally, that is the information a GP can work with.

An example that shows the difference. Two people both have a lymphocyte count of 1.1 ×10⁹/L, just under the lower limit at many laboratories. One was properly ill two weeks earlier, the other was not.

For the first, a temporary dip after a viral infection is a known explanation. For the second, it is a reason to repeat the value after a few weeks and discuss the pattern with a GP.

Same number, different meaning.

In a Danish population study of 98,344 people, an incidentally found low lymphocyte count was associated with a higher risk of infection (PMID 30383787). That is an association at group level. It is explicitly not a prediction for one individual.

What do you notice when your immune system is not working well?

The signals are rarely dramatic. Usually it is infections that return more often than you were used to, last longer, or run unusually heavily. Slow healing cuts and repeated fungal infections can belong to that pattern too.

One caveat: none of those complaints is specific. They fit a busy year, poor sleep, or an allergy you mistake for a cold just as well.

That last one comes up remarkably often. Someone believes they have been continuously congested since March, while the pattern fits pollen far better.

The article on frequent colds explains how to tell those apart.

Sometimes something genuinely does suppress your immunity. Certain medication, or a chronic condition. The piece on a weakened immune system covers that route separately.

Three stubborn myths about resistance

A few ideas circulate around this topic that are simply wrong. They stick because they sound logical. Here are three, each starting with what is actually true.

True: your white blood cells do the work of your immune system. Not true: that a higher count is therefore better. A raised white cell count usually means your body is reacting to something, which is a different thing from being strong.

True: shortfalls in certain nutrients can go together with getting ill more often. Not true: that topping up above a normal value adds anything. Above a certain point, more is just more, not better.

True: stress affects your body, including inflammation. Not true: that you read it off one isolated value. No stress number sums up your immune system, however much people wish it did.

That last one is why I am wary of any page promising to score your resistance. What measurable link there is between stress and blood values is set out in the article on chronic stress and your blood values.

Can you have your immune system tested?

You can have parts measured, not the whole. A full blood count with differential shows how many white cells you have and how they are distributed. Immunoglobulins show whether you produce certain antibody classes. Together they give clues, not a verdict.

The antibody classes are a story of their own. IgM appears first in a new infection, IgG comes later and stays around longer.

What that combination means is explained in the piece on immunoglobulins.

Selective IgA deficiency is described as the most common primary immunodeficiency, and most people who have it never notice (PMID 20101521). It is often found by chance, during testing requested for something else entirely.

If you want several of these values measured at once, the blood count and inflammation markers sit in the extended health checkup. Individual markers can be added by building your own blood test.

How does your immune system change with age?

Your immune system ages along with the rest of you. The adaptive layer responds more slowly on average to something new, while memory of earlier infections largely stays intact. The balance between the different types of white blood cells also shifts slowly in many people.

This happens gradually and differs per person. Age on its own therefore says little about how someone responds to an infection.

What does play a part are the conditions that become more common over the years. Poorly controlled diabetes is one example, and it is exactly why a GP looks at the whole picture instead of a single value.

The Dutch public health institute RIVM tracks which respiratory infections are circulating in the Netherlands and in which period. Part of the seasonal difference people attribute to their own resistance simply comes down to more virus circulating in those weeks.

What lifestyle does to your immune system

Sleep is the best studied factor here. In a study where participants were exposed to a cold virus under controlled conditions, people sleeping under six hours got ill markedly more often than people sleeping over seven hours (PMID 26118561).

The differences were large. Under five hours of sleep, the odds ran roughly four and a half times higher than above seven hours.

Vitamin D is more nuanced. In a meta-analysis of individual participant data, supplementation was associated with a lower risk of acute respiratory tract infection, with the clearest difference among people starting from very low levels (PMID 28202713).

That is again a group level finding. Whether topping up makes sense for you is a conversation with your GP, not something to take from an article.

The link between sleep and your blood values is worked out further in the article on poor sleep and your blood values.

What to do with an abnormal result

One value just outside the reference range is rarely the whole story. Reference ranges differ per laboratory, and they are built so that a proportion of healthy people falls outside them by definition. The trend across several measurements says more than one snapshot.

Take your result to your GP, including when it was measured and what was going on in the weeks before. A recent infection, a new medication or a heavy training block can all shift the picture.

What reference ranges actually are is explained in the article on reference ranges in blood testing. The individual components of your blood count are covered in the piece on the full blood count.

If you have a result you cannot place, book an appointment with your GP and bring the printout. That conversation gets you further than an evening of searching.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Chaplin DD. Overview of the immune response. Journal of Allergy and Clinical Immunology, 2010 (PMID 20176265).
• Warny M, et al. Lymphopenia and risk of infection and infection-related death in 98,344 individuals. PLoS Medicine, 2018 (PMID 30383787).
• Heikkinen T, Jarvinen A. The common cold. The Lancet, 2003 (PMID 12517470).
• Prather AA, et al. Behaviorally assessed sleep and susceptibility to the common cold. Sleep, 2015 (PMID 26118561).
• Yel L. Selective IgA deficiency. Journal of Clinical Immunology, 2010 (PMID 20101521).
• Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections. BMJ, 2017 (PMID 28202713).