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Plenty of people who catch every bug that goes around assume their immune system is simply weak. Sometimes that is true. More often, something specific is dragging it down, and that something is usually identifiable once you know where to look. Immune deficiency covers a wide spectrum, from rare inherited conditions where a single gene fault disables part of the defence system, to far more common acquired problems caused by diet, medication, sleep or another illness. The seven factors below are the ones clinicians see again and again.
1. A Diet Short on Protein, Zinc and Vitamin D
Immune cells are built out of what you eat. White blood cells need protein to divide, zinc to mature, and vitamin D to signal properly. Protein-energy malnutrition remains the most common cause of acquired immunodeficiency worldwide, and it does not have to look like starvation. An older adult living on tea, toast and biscuits can slide into it quietly over eighteen months.
Zinc handles T cell development, and low levels turn up more often than expected in people over 65, in vegetarians eating a lot of phytate-heavy grains, and in anyone with untreated coeliac disease. Vitamin D receptors sit on macrophages and T cells; low winter levels in northern latitudes track with higher rates of respiratory infection. Iron, B12 and folate matter too, mainly for how fast immune cells can multiply when they are needed.
Shortfalls often show up elsewhere first. Hair tends to flag low iron, zinc or protein months before the infections start, so persistent shedding and thinning are worth investigating properly rather than covering up with a new shampoo.
2. Chronic Stress and Habitual Short Sleep
Cortisol is not the villain it is made out to be. It damps inflammation, which is useful when inflammation would otherwise run away with you. The problem is a stress response that never switches off. Sustained high cortisol reduces lymphocyte proliferation and leaves the immune system in a low-grade inflammatory state that handles new infections badly.
Sleep is where the effect is easiest to measure. In a well-known experiment, healthy young men restricted to four hours in bed for a single night showed natural killer cell activity drop by roughly 70%. People who routinely sleep under six hours mount weaker antibody responses to vaccination and are more likely to catch a cold after deliberate exposure to rhinovirus.
Shift work, new parenthood, caring for a relative with dementia and untreated insomnia all produce the same pattern. Seven to nine hours with a consistent wake time does more for your defences than most supplements on the shelf.
3. Ageing and the Shrinking Thymus
The thymus, a small gland behind the breastbone where T cells learn what to attack and what to leave alone, is at its largest around puberty. From then on it shrinks steadily, and by 60 it is mostly fatty tissue. New T cell production falls steeply, and the pool you rely on becomes narrower and slower to react to pathogens it has never met.
This explains why shingles becomes common after 50, why the same flu strain that a 25-year-old shrugs off can put a 75-year-old in hospital, and why vaccines for older adults are often adjuvanted or given at higher doses. Age itself is not modifiable. What you do with it is, and resistance training, adequate protein and tight control of chronic conditions preserve immune function better than any product marketed as an immune booster.
4. Immune-Suppressing Medication and Cancer Treatment
Some of the most effective drugs in modern medicine work precisely by holding the immune system back. The risk rises with dose and duration, but a few categories account for most of it:
- Oral corticosteroids such as prednisone at 20 mg or more daily for longer than two weeks
- Chemotherapy, especially during the neutropenic window 7 to 14 days after a cycle
- Biologics that block TNF, which can reactivate dormant tuberculosis
- Anti-rejection drugs after transplant, and some disease-modifying drugs used for rheumatoid arthritis
If you take any of these, ask about non-live vaccines before you start rather than after, and about prophylactic antibiotics if neutropenia is expected. Never stop a drug on your own to protect your immune system; the flare or relapse is usually worse than the infection risk. Cancer adds its own layer, since some blood cancers crowd normal immune cells out of the bone marrow. Understanding what raises cancer risk and how treatment has changed helps put that into context.
5. Diabetes, Kidney Disease and Other Long-Term Conditions
High blood glucose directly depresses neutrophil function. Someone with an HbA1c above 8% is not only looking at eye and nerve damage; they get more urinary tract infections, more skin infections and slower-healing wounds. Chronic kidney disease causes immune dysfunction through retained toxins and the loss of protective antibodies in urine. Liver disease cuts complement production. Obesity with a BMI over 30 keeps the body in low-grade inflammation that blunts vaccine responses and delays wound repair.
Autoimmune disease complicates the picture because the immune system is overactive but misdirected, and the drugs used to control it suppress the parts you actually need. Symptoms overlap heavily with other conditions, which is a large part of why the early signs of autoimmune disease are so often missed until damage has already occurred.
6. Smoking, Heavy Drinking and Air Pollution
Smoking paralyses the cilia lining your airways, so bacteria and viruses sit in the lungs instead of being swept out, and it reduces natural killer cell activity on top of that. Drink two or more alcoholic drinks a day for years and macrophage function and gut barrier integrity suffer, which is one reason heavy drinkers get more pneumonia and poorer vaccine responses.
Air pollution works on the same tissues. Fine particles under 2.5 micrometres get deep into the alveoli and drive persistent airway inflammation. People living near heavy traffic or working with solvents, welding fumes or silica dust carry a measurable excess of respiratory infections, and the effect is worse if they also smoke.
7. Infections and Conditions That Destroy Immune Tissue
Untreated HIV remains the most important example. It infects and kills CD4 T cells directly, and a count below 200 is the point where opportunistic infections such as pneumocystis pneumonia appear. Measles is another: it causes what researchers call immune amnesia, wiping memory B cells so that protection you built up against other diseases disappears for months to years afterwards.
Removal of the spleen, whether after trauma or for a blood disorder, leaves you without a key filter for encapsulated bacteria such as Streptococcus pneumoniae and Neisseria meningitidis, which is why splenectomy patients need specific vaccinations and prompt antibiotics for any fever. Severe burns and extensive gut damage do similar harm by stripping away physical barriers and immune tissue at once. Spleen removal also raises the odds of thrombosis, and common blood clotting disorders and the signs they cause are worth knowing if you have had the operation.
When Recurrent Infections Deserve a Proper Workup
Most adults who feel they are always ill have normal immune function. The pattern that matters is not how many colds you get but how severe they are, where they sit and what grows from them. Ten to twelve minor infections a year is within normal range for a household with young children.
These are the flags that justify asking your doctor for testing:
- Two or more pneumonias within three years, or any pneumonia needing intravenous antibiotics
- Four or more ear infections, or two or more sinus infections, in a single year
- Recurrent deep skin abscesses or infections that need surgical drainage
- Thrush in the mouth of an otherwise healthy adult
- Infections with unusual organisms, or a family history of primary immunodeficiency
- Poor growth or failure to thrive in a child
Basic screening is straightforward and mostly available through a GP: a full blood count with differential, immunoglobulin levels for IgG, IgA and IgM, vaccine antibody titres, complement studies and an HIV test, with lymphocyte subsets if anything looks abnormal. Keep a simple log of infections, dates, antibiotics and results for six months before your appointment. A clear pattern on paper gets you further in a ten-minute consultation than a memory of always being unwell.
