5 Signs of a Weak Immune System and What to Do
Everyone catches a cold now and then, and getting sick occasionally does not mean your defenses are failing. But when infections become unusually frequent, severe, or hard to shake, the pattern may point to a weak immune system. The signs of a weak immune system are easy to miss or misread because many of them overlap with everyday complaints like tiredness, bloating, or slow-healing cuts. This guide walks through the five most common signs, what causes immune weakness, how doctors actually test immune function, and which evidence-based habits genuinely support your defenses. You will also learn how to tell a normal immune response apart from a genuine warning sign, and when it is worth talking to a doctor.
What Does a Weak Immune System Mean?
Your immune system is a layered defense network made up of white blood cells, antibodies, signaling proteins, and physical barriers like skin and mucous membranes. The innate branch responds fast and broadly to threats, while the adaptive branch learns to recognize specific pathogens and builds targeted weapons, including antibodies, that remember past invaders for years.
When people talk about a weak immune system, they usually mean an underactive one — what doctors call immunodeficiency. In this state, the immune system struggles to recognize or respond to pathogens effectively. The result is a characteristic pattern: infections that happen more often, hit harder, last longer, or are caused by organisms that healthy immune systems normally handle without trouble.
The opposite problem also exists. In an overactive immune system, the defenses turn inward and attack the body's own tissue. This is the basis of autoimmune disorders such as rheumatoid arthritis, lupus, type 1 diabetes, and celiac disease. The distinction matters because the symptoms can overlap — fatigue, for example, is common in both — but the underlying biology and the appropriate treatment are completely different. A weak immune system fails to defend; an autoimmune system attacks by mistake.
So how can you tell if you have a strong or weak immune system? The honest answer is that no single illness tells you much. Most adults catch two to four colds a year, and young children may have six to eight or more as their immune systems learn the microbial world. One rough winter after a stressful stretch or a period of poor sleep does not indicate a defect. What does suggest genuine weakness is a recurring pattern: infections that keep returning, need repeated courses of antibiotics, become serious more often than they should, or take far longer to resolve than expected. Patterns, not single events, are what deserve attention.
5 Signs of a Weak Immune System
No single symptom proves immune weakness, and weak immune system symptoms often overlap with other health issues. The five signs below carry the most weight when they appear together, recur over months, or clearly exceed what is typical. For each one, it helps to know what the sign looks like, why it happens, and at what point it stops being "probably nothing."
Discover the Microbiome Test
ISO-certified EU lab • Sample stays stable during shipping • GDPR-secure data
1. Frequent or Severe Infections
This is the hallmark sign. A concerning pattern looks less like "I caught the office cold" and more like a steady stream of infections that require medical treatment. Clinical criteria used by immunologists to flag possible primary immunodeficiency offer useful thresholds: needing antibiotics four or more times a year, having pneumonia twice within twelve months, suffering chronic or repeated sinus infections, experiencing multiple serious ear infections as an adult, or developing recurrent deep skin infections such as boils or cellulitis. Infections that require intravenous antibiotics, or persistent thrush and other fungal infections, are also red flags.
The mechanism is fairly intuitive. If pathogens repeatedly gain a foothold, survive, and spread, some layer of defense — physical barriers, neutrophils that engulf bacteria, or antibodies that neutralize viruses — may not be doing its job. Severity matters as much as frequency. An immune system that turns a routine cold into bronchitis, or a small skin nick into a spreading infection, is signaling that something in the response chain is lagging, even if the illnesses eventually resolve.
2. Slow Wound Healing
Healing is an immune process from the first moment. When skin breaks, neutrophils and macrophages rush in to clear bacteria and damaged tissue, immune signaling molecules direct the repair sequence, and only then do skin cells close the gap. A minor cut on a healthy adult typically seals within a week or so.
When wounds stay red, swollen, warm, or oozing for more than a week or two, or when small nicks repeatedly fester instead of closing, immune function is one possible explanation — an insufficient cellular response allows bacteria to persist and stalls the repair process. That said, slow healing has other common causes, including diabetes, poor circulation, and smoking, so a persistently slow-healing wound is a reason to see a doctor rather than a self-diagnosis. The practical threshold: if a minor wound is not clearly improving within one to two weeks, get it examined.
3. Persistent Fatigue
Immune activity is expensive. Mounting a response consumes enormous energy, and the signaling molecules that coordinate it — cytokines — deliberately induce the heavy, drained feeling you get during the flu. That fatigue during illness is normal and even useful, because it pushes the body to rest and redirect resources toward defense.
View example recommendations from the InnerBuddies platform
Preview the nutrition, supplement, food diary and food recipe platform recommendations that InnerBuddies can generate based on your gut microbiome test
The concerning version is different: a deep exhaustion that lingers for weeks, is not relieved by sleep, and has no obvious explanation. A chronically strained immune system may keep the body in a state of low-grade activation, or repeated subclinical battles with pathogens may drain reserves. The important caveat is that fatigue is one of the least specific symptoms in medicine. Sleep deprivation, thyroid problems, anemia, depression, and many other conditions cause it too. Persistent fatigue on its own is not evidence of immune weakness — but fatigue combined with frequent infections or slow recovery fits the pattern much more convincingly.
4. Recurring Digestive Problems
The gut is not just a digestive organ; it is one of the largest immune organs in the body. An estimated 70 percent of the immune system's cells sit in and around the intestinal wall, where they constantly sample the microbial environment and coordinate responses. This close partnership explains why immune weakness and digestive symptoms so often travel together.
Regular episodes of gas, cramping, bloating, or diarrhea — particularly frequent diarrhea, or digestive upset that follows every course of antibiotics or every stomach bug — can reflect an imbalance in gut bacteria or an immune system struggling to manage the intestinal environment. Repeated gut infections themselves are also part of the broader "frequent infections" pattern. The honest caveat: irritable bowel syndrome, food intolerances, and many other conditions produce similar symptoms, so digestive trouble alone proves nothing. But if recurring digestive problems appear alongside the other signs here, the gut may be pointing to the underlying story — and understanding your gut microbiome with a gut microbiome test can add context that symptoms alone cannot provide.
5. Prolonged Recovery From Illness
A typical cold in an adult runs seven to ten days and then fades. One of the clearest signs of immune weakness is when recovery consistently overruns that timeline: a cough that hangs on for three or four weeks, fatigue that persists long after other symptoms clear, or infections that seem to chain together — a cold that becomes sinusitis, which then slides into a chest infection, each requiring another course of medication.
Comparison is a useful informal tool. If family members or colleagues caught the same virus and were back to normal in a week while you were still unwell a month later, your recovery process — not just your exposure — may be the differentiator. The immune system's job after an infection is cleanup and consolidation, clearing debris, finishing the antibody response, and standing down inflammation. When that final phase drags on repeatedly, it suggests the response itself is running below par.
What Causes a Weak Immune System?
The question of what causes a weak immune system has two broad answers: causes you are born with and causes you acquire along the way. Doctors separate them because the implications — for testing, treatment, and outlook — are quite different.
Primary (Inherited) Immunodeficiency
Primary immunodeficiencies are genetic conditions present from birth, and there are more than 400 recognized types. They range from relatively mild issues, such as selective IgA deficiency, in which the body makes too little of one antibody type, to severe combined immunodeficiency, in which major parts of the adaptive immune system never develop. Common variable immunodeficiency (CVID) is one of the more frequent forms in adults; it can go unnoticed for decades before recurrent sinus and lung infections finally prompt testing. Some primary immunodeficiencies announce themselves in infancy, while others surface quietly in adulthood, which is why a family history of unexplained recurrent infections is always worth mentioning to a doctor.
Medical Conditions and Treatments
Secondary causes are far more common, and several are serious medical conditions. HIV attacks and destroys CD4 T cells, the coordinators of the adaptive immune response; without treatment, this destruction can progress to AIDS. Cancers of the blood and bone marrow, such as leukemia and lymphoma, crowd out the cells from which white blood cells are made. Chemotherapy and radiation, by design, suppress bone marrow activity, which temporarily lowers immune defenses.
Medications can also weaken immunity, sometimes intentionally. Corticosteroids, transplant anti-rejection drugs, and biologic therapies for autoimmune diseases all dampen immune activity, leaving people more susceptible to infection as a known trade-off. Chronic illnesses including diabetes and kidney disease can blunt immune responses over time, and people who have had their spleen removed carry a lifelong elevated risk from certain encapsulated bacteria.
Lifestyle and Nutritional Factors
Day-to-day factors influence immune competence more than most people realize. Malnutrition remains the leading cause of immunodeficiency worldwide, because producing antibodies and immune cells requires a steady supply of protein and micronutrients. Deficiencies in specific nutrients matter too: vitamin D helps regulate immune responses, zinc supports immune cell development and signaling, and shortages of iron, folate, or vitamin B12 can impair the production and function of blood and immune cells.
2-minute self-check Is a gut microbiome test useful for you? Answer a few quick questions and find out if a microbiome test is actually useful for you. ✔ Takes 2 minutes ✔ Based on your symptoms & lifestyle ✔ Clear yes/no recommendation Check if a test is right for me →Chronic stress is another well-documented factor. Prolonged elevation of stress hormones such as cortisol suppresses lymphocyte activity and antibody production, and research links ongoing psychological stress with higher rates of respiratory infection. Sleep deprivation works along similar pathways — the immune system performs much of its regulatory housekeeping during deep sleep. Excess alcohol and smoking add further wear, and aging gradually blunts immune responses in a process called immunosenescence. In real life, these factors rarely act alone; poor sleep amplifies stress, stress degrades diet, and the combined load is what the immune system ultimately feels.
How Is a Weak Immune System Tested and Diagnosed?
Symptoms can raise suspicion, but a weak immune system test involves actual measurement. There is no single scan for immune strength; instead, doctors build a picture from your infection history, a physical exam, and targeted laboratory work.
The usual starting point is a complete blood count with differential, which measures the total number of white blood cells and breaks them down by type — neutrophils, lymphocytes, monocytes, and others. Abnormally low or imbalanced counts provide the first objective clue. Next come immunoglobulin levels: blood tests that quantify IgG, IgA, and IgM, the antibodies that do most of the work of neutralizing viruses and bacteria. Low levels point toward antibody deficiencies such as CVID.
A particularly informative test is the vaccine antibody titer. Because vaccination is a controlled, known immune challenge, measuring antibodies to vaccines you have already received — such as tetanus or pneumococcal vaccines — shows how well your adaptive immune system formed and maintained a memory response. Additional tests may include complement proteins, an HIV test, and, when a primary immunodeficiency is suspected, specialized genetic panels.
When initial results or the infection pattern warrant it, doctors refer patients to an immunologist, a specialist in immune system disorders, for deeper evaluation. Two honest limitations deserve mention. First, normal results do not always exclude a subtle immune problem, which is why the clinical history carries so much weight. Second, not everyone who feels run-down needs this workup; testing is most valuable when the red-flag patterns described below are present. It is also worth understanding that a gut microbiome test, while useful for educational insight, is not a diagnostic test for immunodeficiency and does not replace medical evaluation.
Signs Your Body Is Fighting a Virus vs. Signs of Weakness
One of the most common misinterpretations is reading a normal immune response as a malfunction. When a virus enters your body, the symptoms you feel are largely the immune system at work: a mild fever raises body temperature to slow viral replication, cytokines cause the aches and drowsiness that push you to rest, mucus production traps and flushes pathogens, and swollen glands in the neck signal that lymphocytes are actively proliferating. These are signs your body is fighting a virus — not signs that it is losing.
| What you notice | Normal immune response | Possible sign of weakness |
|---|---|---|
| Fever | Mild fever for two to three days that settles on its own | Repeated fevers without a clear infection, or a fever that keeps returning |
| Illness length | Colds and mild flu resolve within about seven to ten days | Symptoms linger for weeks, fade, then relapse |
| Frequency | Two to four mild colds per year in adults | Back-to-back infections, or four or more per year requiring antibiotics |
| Severity | Uncomfortable but manageable at home | Common infections repeatedly escalate into sinusitis, bronchitis, or pneumonia |
| Recovery | Energy returns within a week or two after symptoms clear | Exhaustion and symptoms persist far longer than the illness should last |
A separate category of confusion involves signs of an overactive immune system. Persistent joint pain and stiffness, skin rashes, dry eyes or dry mouth, unexplained weight change, and consistently cold hands or fingers can reflect autoimmune activity or inflammation rather than a failing defense. These symptoms are just as deserving of medical attention — they simply point the investigation in a different direction. Noting which category your pattern fits into helps your doctor ask better questions from the very first appointment.
Your Gut Microbiome and Immune Strength
Because so much immune tissue is concentrated in the gut, the trillions of bacteria living there — the gut microbiome — are in constant dialogue with the immune system. Research suggests this relationship works in several ways: gut bacteria help train developing immune cells to respond appropriately to threats, ferment dietary fiber into short-chain fatty acids that support immune regulation and the integrity of the gut barrier, and compete with harmful microbes for space and resources. In effect, the microbiome acts as a daily training ground that helps calibrate immune responses.
Studies have linked reduced microbial diversity and other markers of imbalance with increased inflammation and greater susceptibility to infections. An important nuance is that much of this research shows association rather than proven causation — dysbiosis may contribute to immune problems, result from them, or both. Individual variability is enormous. Your microbiome is shaped by your birth history, diet, antibiotic use, environment, and lifestyle, and it can shift noticeably within weeks when those inputs change.
This is where measurement becomes useful. Symptoms like bloating or frequent infections suggest that something is off, but they cannot tell you what. A personalized microbiome analysis can reveal the composition and diversity of your gut bacteria and how your profile compares with general population patterns — context that may help you understand why you feel the way you do and which dietary directions are worth exploring. What such testing cannot do is diagnose immunodeficiency or any specific disease, and results always need interpretation alongside your overall health picture. Think of it as adding a layer of personalized understanding, not a medical verdict.
Can You Fix a Weak Immune System? Evidence-Based Steps
Whether a weak immune system can be "fixed" depends on the cause. Lifestyle-related weakening, driven by sleep debt, chronic stress, poor diet, or nutrient deficiencies, often improves meaningfully once those inputs change. Primary immunodeficiencies are managed rather than cured, typically under specialist care, sometimes with antibody replacement therapy. For everyone else, the honest framing is not "boosting" immunity — the immune system is not a muscle you can grow on demand — but giving it the conditions it needs to function well. Here is what the evidence supports:
Become a member of the InnerBuddies community
Perform a gut microbiome test every couple of months and view your progress while following-up on our recommendations
- Prioritize sleep. Aim for seven to nine hours per night. One widely cited study found that people sleeping fewer than six hours were roughly four times more likely to catch a cold after viral exposure than those sleeping more than seven. Sleep is when much of immune regulation and memory consolidation happens.
- Manage chronic stress. Prolonged stress hormones measurably suppress immune cell function. Regular exercise, mindfulness practice, time outdoors, and maintaining social connections are all reasonable, evidence-aligned tools — not because they "boost" anything, but because they lower a suppressive burden.
- Eat for immune support. Adequate protein, a wide variety of vegetables and fruits, and plenty of fiber feed both your immune cells and your gut bacteria. Fermented foods can add microbial variety. No single superfood matters as much as the overall pattern, and malnutrition clearly degrades immunity.
- Exercise moderately and consistently. Regular moderate activity is associated with fewer respiratory infections, partly by improving immune surveillance. Extreme, sustained overtraining can transiently suppress immunity, so balance matters.
- Stay up to date with vaccinations. Vaccines train the adaptive immune system safely and are the most direct way to prepare your defenses against specific serious pathogens. If you are immunocompromised, discuss timing and vaccine types with your doctor, since some live vaccines require special caution.
- Correct deficiencies, not guesses. If you suspect low vitamin D or zinc, testing is more reliable than supplementing blind. Restoring a true deficiency supports immune function; taking megadoses beyond adequacy has not been shown to add benefit and can cause harm.
- Handle the basics. Handwashing, food safety, avoiding smoking, and limiting alcohol reduce the pathogen load and physiological wear your immune system faces every day.
| Approach | What the evidence suggests |
|---|---|
| Sleep (7–9 hours) | Consistently linked with lower infection risk; short sleep associated with markedly higher cold susceptibility |
| Balanced, fiber-rich diet | Supports immune cell function and gut microbial diversity; malnutrition clearly impairs immunity |
| Moderate regular exercise | Associated with fewer respiratory infections; extreme overtraining may temporarily suppress immunity |
| Vaccination | Trains adaptive immunity safely; the most proven way to prepare for specific pathogens |
| Vitamin D | Correcting deficiency supports normal immune function; benefits beyond adequacy are not established |
| Zinc | Deficiency impairs immunity; lozenges taken soon after symptoms start may modestly shorten colds |
| Vitamin C | Does not prevent colds in the general population; regular use may slightly shorten duration for some people |
| "Immune-boosting" pills and detoxes | No evidence of overnight fixes; megadoses of some supplements carry real risks |
The myths deserve a direct word. Products promising to "supercharge" your immune system overnight misrepresent how immunity works — the system is a balance of many cell types and signals, and indiscriminate stimulation is not desirable or possible through a pill. If you want to know how your current diet and lifestyle may be shaping the gut environment that helps train your immune system, understanding your gut microbiome offers a starting point grounded in your own data rather than marketing claims. Real change comes from sustained habits, and those habits take weeks to months to shift immune outcomes.
When to See a Doctor
Occasional mild illness managed at home is normal life. But certain patterns cross a clear line and justify medical evaluation. Seek an appointment if you experience:
- Four or more infections per year that require antibiotics
- Pneumonia twice within a twelve-month period
- Recurrent deep skin abscesses or persistent skin infections
- Persistent thrush or recurring fungal infections of the skin, mouth, or nails
- Any infection requiring intravenous antibiotics or hospital admission
- Common infections that are unusually severe, unusually long-lasting, or unusually resistant to treatment
- A family history of diagnosed immunodeficiency or unexplained early deaths from infection
Additional general red flags — unexplained weight loss, drenching night sweats, or persistent fevers — also warrant prompt evaluation regardless of infection history. For parents, poor weight gain combined with recurrent infections in an infant is a specific warning sign that pediatricians take seriously.
Knowing what to expect can make the appointment easier. Your doctor will take a detailed infection history, examine you, and typically order first-line blood tests such as a complete blood count and immunoglobulin levels. Depending on the results, referral to an immunologist may follow for more specialized testing. Going in with a written summary of your infections — what, when, how long, and how treated — makes the pattern far easier for your doctor to assess.
This article is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional about symptoms or before making changes that affect your health.
Key Takeaways
- Getting sick a few times a year is normal; patterns of frequent, severe, or unusually prolonged infections are what suggest immune weakness.
- The five most consistent signs are frequent infections, slow wound healing, persistent fatigue, recurring digestive problems, and prolonged recovery from illness.
- Concrete thresholds — needing antibiotics four or more times a year, or pneumonia twice in twelve months — warrant medical evaluation.
- A weak (underactive) immune system is different from an overactive one; joint pain, rashes, and cold hands often point toward autoimmune activity instead.
- Causes range from inherited immunodeficiencies and medical treatments to sleep deprivation, chronic stress, and deficiencies in nutrients such as vitamin D and zinc.
- Doctors assess immune function with blood tests — white cell counts, immunoglobulin levels, and vaccine antibody responses — rather than symptoms alone.
- Evidence-based support includes adequate sleep, a balanced diet, moderate exercise, stress management, and recommended vaccinations; "immune-boosting" quick fixes are not supported by evidence.
- The gut microbiome interacts closely with the immune system, and understanding your individual gut profile may add context that symptoms alone cannot provide.
Frequently Asked Questions
What are two diseases that destroy the immune system?
HIV is the classic example: it attacks and destroys CD4 T cells, the coordinators of adaptive immunity, and without treatment can progress to AIDS. Untreated blood cancers such as leukemia and lymphoma can also progressively crowd out the bone marrow cells that produce white blood cells. Both conditions require specialist care, and effective treatments exist, particularly for HIV.
Can you fix a weak immune system?
It depends on the cause. Weakness linked to sleep deprivation, chronic stress, or nutrient deficiencies often improves substantially when those factors are addressed, while primary immunodeficiencies are managed long-term rather than cured, typically under an immunologist's care. In both cases, sleep, nutrition, stress management, and vaccination support whatever immune function you have.
How can I tell if I have a strong or weak immune system?
Look at patterns over time rather than any single illness. A few mild colds a year that resolve in about a week suggest normal function, while frequent, severe, or long-lasting infections — especially those requiring repeated antibiotics — deserve a doctor's attention. Blood tests can measure what symptoms can only hint at.
What are the signs that my body is fighting a virus?
A mild fever, fatigue, body aches, runny nose, sore throat, and swollen glands all reflect immune activity — white blood cells releasing signaling molecules that coordinate the response. These symptoms usually peak within two to three days and fade within a week or so. Feeling unwell during an infection is generally a sign your defenses are working, not failing.
Is a weak immune system dangerous?
It can be, depending on severity. Mild, lifestyle-related weakening mainly raises the risk of more frequent or longer infections, while significant immunodeficiency can allow serious complications such as pneumonia or infections from organisms a healthy immune system would easily control. This is why persistent red-flag patterns deserve proper medical evaluation rather than self-management.
2-minute self-check Is a gut microbiome test useful for you? Answer a few quick questions and find out if a microbiome test is actually useful for you. ✔ Takes 2 minutes ✔ Based on your symptoms & lifestyle ✔ Clear yes/no recommendation Check if a test is right for me →Do women show different signs of weak immunity?
The core signs — frequent infections, slow healing, prolonged recovery — are broadly similar across sexes. However, research suggests around 80 percent of people with autoimmune conditions are women, so symptoms like joint pain, rashes, fatigue, and cold hands in women more often reflect an overactive immune system than a weak one. Hormonal shifts through the menstrual cycle, pregnancy, and menopause can also influence immune responses.
What blood test checks your immune system?
Doctors commonly start with a complete blood count with differential, which measures white blood cell numbers and types, followed by immunoglobulin tests that quantify IgG, IgA, and IgM antibody levels. Vaccine antibody titers show how well your immune system responded to vaccines you have received. An immunologist may arrange more specialized testing if initial results or your infection history warrant it.
How many colds per year are normal?
Most adults get two to four colds a year, and young children in daycare or school may have six to eight or more. Colds typically last seven to ten days and resolve without treatment. Several mild colds a year is not a sign of weakness as long as recovery is normal and infections do not escalate.
Does stress really weaken the immune system?
Chronic stress appears to. Prolonged elevation of stress hormones such as cortisol suppresses white blood cell activity and antibody production, and research links ongoing psychological stress with higher rates of respiratory infection. Short-term acute stress has more mixed and less clear effects on immunity.
Can gut health affect immune strength?
Yes. A large share of the body's immune cells reside in and around the gut, where they constantly interact with gut bacteria. The microbiome helps train immune cells, produces beneficial compounds from dietary fiber, and supports the gut barrier, and studies associate microbial imbalance with inflammation and infection risk. The relationship is complex and varies considerably between individuals.
Do vitamins or supplements help a weak immune system?
Correcting a genuine deficiency — such as low vitamin D or zinc — supports normal immune function, but supplements do not boost immunity in people who are already adequate, and megadoses can be harmful. Vitamin C has not been shown to prevent colds in the general population, though regular use may slightly shorten their duration for some. Testing for deficiencies before supplementing is the most reliable approach.
Can children have a weak immune system?
Children naturally experience more infections than adults because their immune systems are still learning, so frequent mild colds alone are not alarming. Red flags include pneumonia twice in a year, infections requiring IV antibiotics, persistent thrush, poor weight gain, or a family history of immunodeficiency. A pediatrician can arrange appropriate testing when these patterns appear.
Conclusion: What the Signs Really Tell You
The signs of a weak immune system are less about any single bad cold and more about the story your health tells over months: infections that return, wounds that linger, fatigue that does not lift, digestive trouble that keeps recurring, and recoveries that stretch far beyond normal. Concrete thresholds — four antibiotic-requiring infections a year, pneumonia twice in twelve months, infections that turn serious — turn vague worry into a clear reason to see a doctor.
Equally important is knowing what these signs do not mean. Fatigue can come from a thousand causes, digestive symptoms can reflect many conditions, and a fever during the flu is your immune system working, not failing. Symptoms raise questions; blood tests measuring white cell counts and immunoglobulin levels are what begin to answer them. And because so much immune activity is coordinated in the gut, personalized insight from microbiome testing may add a useful layer of understanding about the environment your immune system lives in — as context alongside, never instead of, medical care. Every immune system is different, and the most reliable path forward combines honest observation of your own patterns with evidence-based habits and professional evaluation when the pattern warrants it.
Keywords
signs of a weak immune system, weak immune system symptoms, frequent infections, slow wound healing, persistent fatigue, digestive problems, recurrent colds and flu, chronic sinus infections, what causes a weak immune system, immunodeficiency, autoimmune disorder, white blood cells, antibodies, immunoglobulin levels, weak immune system test, blood test for immunity, immunologist, pneumonia, skin infections, HIV, chemotherapy, malnutrition, vitamin D deficiency, zinc, chronic stress, sleep and immunity, stress and immune function, gut microbiome, immune response, how to strengthen immune system, when to see a doctor, immunocompromised