Weakened immune system: causes and which blood values give insight

Four colds since January, two of them with fever, and a throat infection in July on top. People arrive with that list and one question: is my immune system weakened?

There is no blood test that gives your immunity a grade. That honest answer rarely sits at the top of the search results. Blood can show pieces of the picture, and it can make a few known causes of reduced immunity visible.

What strikes me about the Dutch search results: nearly every page ends at a jar of supplements. Almost none name the two routes that matter most in adults. Those are medication that suppresses the immune response, and a chronic illness that is poorly controlled.

We stay with those acquired causes here. Inborn immune disorders and autoimmune disease are a different story and belong with a doctor.

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

You notice a pattern, not a single symptom. Infections return more often, last longer or run heavier than you are used to in yourself. Adults average two to four colds a year, and that is simply normal (PMID 12517470). The pattern around the number says more than the number.

Signals a GP listens for:

• infections that keep coming back in the same place, such as your lungs or sinuses
• an infection that only clears with antibiotics, time after time
• recovery that structurally takes weeks longer than it used to
• fever, unintended weight loss or night sweats alongside it

That last combination does not belong in an internet search. Call your GP with it.

Being tired on its own says little about your immunity. Why a run of colds often means something different from what people assume sits in our article on frequent colds and your immune system.

Which causes make your immunity less strong?

In adults four routes come up most often: medication that suppresses immune function, a chronic illness that is not well controlled, long-term sleep loss or stress, and shortfalls through diet, absorption or alcohol. They act at different points, and they are not equally measurable.

The right-hand column is worded cautiously on purpose. Which investigation fits you is decided by a doctor on the basis of your story, not by a table on the internet.

Medication sits at the top because these drugs do what they are meant to do. Corticosteroids and immunosuppressants damp part of the immune response, almost always to treat something else. That is a trade-off your doctor makes together with you.

Never stop or change a prescribed medication on your own. If you want to look up what a drug does, the Farmacotherapeutisch Kompas is a public Dutch reference for exactly that. Your follow-up question belongs with the doctor who prescribed it.

Chronic illness is the second route. In a large British cohort study, people with type 1 and type 2 diabetes had more infections than the general population (PMID 29330152). The difference was largest for infections that needed hospital treatment.

That is an association at group level. It predicts nothing about one person, and it says nothing about how well someone is managed.

Long-term stress belongs in the same list. What shows up in your blood from it, and what mostly does not, is described in our piece on chronic stress and your blood values.

Alcohol is the forgotten fourth. Heavy and regular drinking is associated with a higher chance of respiratory infections, and it often travels with worse sleep and a less varied diet.

What is a disorder of the immune system?

An immune disorder means one part of your immune system structurally works less well. Doctors split these roughly into two groups: inborn (primary) and acquired later (secondary). That second group is by far the larger one, and medication and illness are its best-known causes.

Your immunity has two layers that work together. Innate immunity reacts within minutes and has no memory. Adaptive immunity starts slower, but builds targeted antibodies and memory cells (PMID 20176265).

A disorder can sit in either layer. That is exactly why no single test covers them both.

The best-known inborn variant is selective IgA deficiency, the most common primary immunodeficiency (PMID 20101521). Many people with this trait have no symptoms at all and find out by chance. How those antibody classes work is explained in our piece on what IgG, IgA and IgM say about your immunity.

Online, the phrase "immune system disease" gets used for immune disorders and for autoimmune disease interchangeably. Those are two different things. In the first your immunity does too little, in the second it turns on your own tissue.

Can you test your immune system?

Not as one score, but partly. There is no blood value that grades your immunity. What blood can contribute: a breakdown of your white blood cells, the antibody classes IgG, IgA and IgM, an inflammation marker, and a few values tied to deficiencies.

The leukocyte differential shows how your white blood cells divide across lymphocytes, neutrophils, monocytes, eosinophils and basophils. It stays a snapshot. An infection from two weeks ago can still colour the picture.

An inflammation marker such as CRP says something about activity in your body, not about the strength of your immunity. What a mildly raised value over a longer period can mean sits in our article on chronic low-grade inflammation.

In our experience this is the moment people get disappointed. They hope for one number that says how strong they are. That number does not exist, and I read every page implying otherwise as sales copy.

What these values are and are not meant for is worked out further in our overview of how your immune system works and what your blood shows.

How much does sleep really matter?

More than most people think, and this is one of the few places with hard numbers. In an American study, sleep was measured for a week in 164 healthy adults. They were then given a cold virus through the nose (PMID 26118561).

Picture two people from that study who slept 7.5 and 5.5 hours a night on average. The short sleeper went on to fall genuinely ill far more often.

The odds ratio came out at 4.24 for five to six hours of sleep, and 4.50 for under five hours (PMID 26118561). That is roughly four times the odds, compared with people sleeping more than seven hours.

I like setting those numbers next to the promise on a box of immunity supplements. Hardly any jar can show an effect of that size.

This was a controlled study in healthy volunteers. It does not prove your cold came from your sleep. It does show how large the influence of sleep can be, and how sparing the rest of the internet is with numbers like these.

Vitamin D is more nuanced. A large analysis of randomised trials found a modest average effect of supplementation on respiratory infections, mainly in people who started with a low level (PMID 28202713). That is not a prompt to start taking anything: that conversation belongs with your GP.

What do you do if you suspect poor immunity?

You start by describing, not by testing. A GP wants to know how many infections you had, where they sat, how long they lasted and whether antibiotics were involved. That history steers which investigation makes sense, far more than one isolated blood value does.

What you can bring to that conversation:

• your infections over the past year, with month and duration
• your current medications, including courses you have just finished
• your average sleep duration over recent months
• conditions that run in your family

Thuisarts, the Dutch patient information GPs themselves refer to, describes when recurring infections deserve further work-up. RIVM, the Dutch public health institute, also tracks which respiratory viruses are circulating. That explains why some winters simply feel heavier than others.

If you want blood values to add context to your story, that can run through an extended health checkup, for example. Read such a result as an addition to your own account, not as a replacement for it.

Concretely: write down your infections from the past year with dates and duration, add your medications, note your average sleep duration and put that list in front of your GP. Ask which next step makes sense in your situation.

References

1. Chaplin DD. Overview of the immune response. J Allergy Clin Immunol. 2010;125(2 Suppl 2):S3-S23. PMID 20176265.
2. Carey IM, Critchley JA, DeWilde S, et al. Risk of Infection in Type 1 and Type 2 Diabetes Compared With the General Population: A Matched Cohort Study. Diabetes Care. 2018;41(3):513-521. PMID 29330152.
3. Prather AA, Janicki-Deverts D, Hall MH, Cohen S. Behaviorally Assessed Sleep and Susceptibility to the Common Cold. Sleep. 2015;38(9):1353-1359. PMID 26118561.
4. Heikkinen T, Jarvinen A. The common cold. Lancet. 2003;361(9351):51-59. PMID 12517470.
5. Yel L. Selective IgA deficiency. J Clin Immunol. 2010;30(1):10-16. PMID 20101521.
6. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. PMID 28202713.
7. Thuisarts, RIVM and Farmacotherapeutisch Kompas. Public Dutch information on infections, respiratory viruses and medicines. Available via thuisarts.nl, rivm.nl and farmacotherapeutischkompas.nl.

Every blood test result at Vitalcheck includes a professional assessment from a BIG-registered doctor. A blood value is not a diagnosis: for treatment decisions, discuss your results with your GP.