Catching every cold, always recovering slower than everyone else, never quite feeling 100%. If your immune system is underperforming, the cause is almost always something measurable and fixable: a nutrient deficiency, a depleted immune marker, or chronic inflammation draining your reserves. A blood test finds which one.
When you tell your GP you keep getting ill, they run a full blood count. That checks your white cell count: the number of immune cells. It does not check whether those cells have the fuel, the antibodies, or the signalling environment they need to actually fight infections effectively. The number of soldiers matters less than whether they're fed, armed, and coordinated.
Your immune army. The FBC shows how many you have and the balance between neutrophils (bacterial defence), lymphocytes (viral defence), monocytes, eosinophils, and basophils. This is what the NHS checks. It's necessary but not sufficient. A normal white cell count with depleted nutrients or low antibodies still means a weakened immune system.
Your antibodies. IgA protects mucosal surfaces (respiratory tract, gut). IgG is your main blood antibody. IgM is your first-response antibody. Low levels of any of these mean reduced capacity to fight specific infections. IgA deficiency (1 in 500 to 700 people) is the most common primary immunodeficiency and directly increases susceptibility to respiratory and gastrointestinal infections. The NHS does not routinely test immunoglobulins unless a specialist requests it.
The fuel your immune system runs on. Vitamin D regulates immune cell activation and has direct antimicrobial effects. Iron powers neutrophil killing capacity. Zinc is essential for T-cell maturation and function. B12 and folate support immune cell production. Deficiency in any one of these weakens the entire system. Most people who feel "run down" have multiple borderline deficiencies compounding each other.
Chronic low-grade inflammation (measured by hs-CRP) exhausts the immune system. It keeps your defences in a state of constant low-level activation, leaving fewer resources available to fight actual infections when they arrive. It's the difference between an army on permanent standby (eventually exhausted) and one that mobilises only when needed (effective). The NHS does not routinely test CRP for "run down" patients.
Your thyroid and hormonal system modulate immune function indirectly. Hypothyroidism slows immune cell production and response time. Low cortisol impairs the inflammatory resolution process. These are rarely considered as immune-relevant by GPs but they affect how efficiently your immune system responds and recovers. A comprehensive panel includes them because the immune system does not operate in isolation from the rest of your body.
This is one of the clearest differences between a mid-range panel and a comprehensive one. Immunoglobulins (IgA, IgG, IgM, IgE) are your actual antibody levels, the proteins your immune system produces to neutralise specific pathogens. They are the most direct measure of immune capacity available from a standard blood draw.
Medichecks does not include immunoglobulins in their standard panels. Forth does not. Thriva does not. Most providers test white blood cells and vitamins but not the antibodies themselves. The TrueVitals Ultimate panel includes the full immunoglobulin profile (IgA, IgG, IgM, IgE) alongside complement proteins (C3, C4), rheumatoid factor, and Anti-CCP. This is the immune depth that catches immunodeficiency, autoimmune processes, and allergic predisposition in a single test.
For context: selective IgA deficiency affects approximately 1 in 500 to 700 people. Most never know. They experience frequent respiratory infections, recurrent sinusitis, and gut problems, and attribute it to "just catching everything that goes around." A single immunoglobulin test identifies it. Without the test, the diagnosis can take over 7 years.
If your immunoglobulins come back low, your TrueVitals report will flag this clearly and recommend discussing the result with your GP. Low immunoglobulins warrant specialist referral and potentially immunoglobulin replacement therapy. This is exactly the kind of finding that changes a life trajectory, and it requires comprehensive testing to uncover.
The immune supplement market in the UK is enormous. Vitamin C, vitamin D, zinc, elderberry, echinacea. Most people who feel run down buy a handful of these hoping something works. Some do. Most don't. Some are actively counterproductive.
Excess iron is toxic. Excess zinc depletes copper. High-dose vitamin D without checking your baseline can push levels too high. Vitamin C supplementation beyond what your body can absorb is simply excreted. Taking all of them when you're only deficient in one is expensive and potentially harmful.
A blood test tells you exactly which nutrients you're deficient in and which you're not. It turns supplementation from guesswork into precision. If your vitamin D is 35 nmol/L, you need 2,000 to 4,000 IU daily. If it's 85 nmol/L, you don't need any. If your ferritin is 18, you need iron with vitamin C for absorption. If it's 90, iron supplementation would be harmful. The test pays for itself by eliminating the supplements you don't need.
Everyone gets ill occasionally. That's normal. What isn't normal:
More than 3 to 4 infections per year that require antibiotics, or infections that take significantly longer to clear than expected
Persistent fatigue alongside frequent illness, which suggests an underlying deficiency or immune dysfunction rather than just bad luck
Recurring infections in the same location (sinuses, chest, throat, urinary tract) which may indicate a specific immune gap such as low IgA
Family members in the same household don't get ill as often, suggesting your susceptibility is individual rather than environmental
Post-viral fatigue that lingers for weeks or months after a cold, flu, or COVID infection, potentially indicating depleted vitamin D, ferritin, or ongoing inflammation
If any of these describe you, a comprehensive blood test is a logical first step. It either identifies a specific, correctable cause or gives you clean data to take to your GP for further investigation. Either way, you stop guessing.
114 biomarkers including full immunoglobulin panel, vitamin D, iron, B12, and inflammatory markers. The immune depth most providers don't offer. Results in 48 hours.
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