Anatomy
The immune system includes bone marrow, thymus, spleen, lymph nodes, tonsils and networks of white blood cells that circulate through the blood and lymphatic system.
Organ Health · Infection & Immunity
Infection & Immunity
Understand how the immune system detects and responds to infection, and how laboratory markers such as CRP, ESR and white blood cell counts support assessment.
Educational information only. Detectives Health helps explain laboratory tests and organ health in plain English. It does not diagnose disease, prescribe treatment or replace professional medical advice. Please discuss your individual results with a qualified healthcare professional.
Understanding the organ
Anatomy, function and importance
Function
The immune system identifies and neutralises bacteria, viruses, fungi and other threats while distinguishing them from the body's own healthy tissues.
Why it matters
A well-functioning immune system supports recovery from infection, wound healing and long-term health.
How it works
A simple explanation in plain English
White blood cells patrol the blood and tissues. When they encounter a threat they release chemical signals that recruit reinforcements.
Some cells engulf invaders directly, while others produce antibodies that mark them for destruction.
Once infection is cleared, memory cells remain to respond faster if the same threat returns.
Inflammation is the immune system's alarm signal — helpful in short bursts, but potentially harmful when persistent.
Laboratory tests
Which laboratory tests assess this organ
Each test contributes a small piece of information. Results are always interpreted alongside symptoms, medical history and other investigations.
CRP (C-Reactive Protein)
- What it measures
- A protein made by the liver that rises rapidly in inflammation or infection.
- Why it is requested
- Widely used to assess active inflammation and monitor response to treatment.
ESR (Erythrocyte Sedimentation Rate)
- What it measures
- How quickly red cells settle — a slower marker of inflammation.
- Why it is requested
- Used alongside CRP, particularly in chronic inflammation.
Procalcitonin
- What it measures
- A marker that tends to rise more in bacterial infections.
- Why it is requested
- Sometimes used in hospital settings to support antibiotic decisions.
White Blood Cell Count
- What it measures
- The total number of infection-fighting cells in the blood.
- Why it is requested
- Reviewed alongside symptoms and other markers to assess infection and immune function.
Neutrophils
- What it measures
- White cells that respond rapidly to bacterial infection.
- Why it is requested
- A key part of the differential white cell count.
Lymphocytes
- What it measures
- White cells central to viral defence and long-term immunity.
- Why it is requested
- Interpreted alongside neutrophils to understand the immune response.
Factors that influence results
Why results vary between people and over time
Laboratory values are shaped by many day-to-day factors. Understanding them helps you and your healthcare professional interpret results in context.
Age
Both very young and older adults have different immune profiles.
Recent infection
Recent illness may leave markers raised for a period.
Vaccination
Vaccines temporarily activate the immune system.
Chronic conditions
Autoimmune conditions, diabetes and kidney disease may influence markers.
Medication
Steroids and immunosuppressants may affect white cell counts and inflammation markers.
Stress
Chronic stress may influence immune function.
Nutrition
Micronutrients such as vitamin D, zinc and vitamin C support immune function.
Sleep
Regular good-quality sleep is essential for immune balance.
Preparing for your blood test
Practical educational guidance
Preparation varies between laboratory tests. Always follow the specific instructions from the clinician or laboratory that requested your sample.
- Follow any fasting instructions given by the requesting clinician — some tests such as fasting glucose or a lipid profile need 8–12 hours without food.
- Take regular medication as usual unless your healthcare professional advises otherwise.
- Mention any supplements — biotin, high-dose vitamins and herbal remedies can influence several laboratory tests.
- Stay well hydrated with water in the hours before your test; dehydration can affect several markers.
- Avoid unusually strenuous exercise in the 24 hours before testing, as this may temporarily alter some results.
- Where possible, attend at a consistent time of day — several hormones and enzymes follow daily rhythms.
- Postpone routine testing if you have an acute infection, unless your clinician specifically wants a result during illness.
Healthy lifestyle
Everyday habits that support this organ
Consistent healthy habits may support organ function over time. They do not replace medical assessment when concerns arise.
Sleep well
Consistent sleep supports the immune response and recovery.
Balanced nutrition
A varied diet provides the micronutrients needed for immune function.
Regular movement
Moderate exercise supports circulation of immune cells.
Hand hygiene
Regular handwashing reduces the transmission of many infections.
Vaccinations
Discuss recommended vaccinations with your healthcare professional.
Manage stress
Long-term stress management supports immune balance.
Do not smoke
Smoking weakens local defences in the airways.
Careful antibiotic use
Only take antibiotics when prescribed and complete the full course.
Symptoms to discuss
Changes worth mentioning to a healthcare professional
These symptoms do not confirm any diagnosis. They are educational prompts for a professional assessment when they are new, persistent or worsening.
- • Persistent fever
- • Recurrent infections
- • Unexplained fatigue with swollen lymph nodes
- • Slow-healing wounds
- • Persistent night sweats
- • Unexplained weight loss
- • Persistent sore throat or cough
- • Persistent joint pain or swelling
Seek urgent medical assessment
In the UK call 999 for emergencies, or 111 for urgent advice. If you experience any of the following, seek help without delay:
- • High fever with confusion, severe headache or a non-blanching rash
- • Rapid breathing, low blood pressure or collapse
- • Severe abdominal pain with fever
- • Any signs of sepsis such as slurred speech, extreme shivering or mottled skin
Routine monitoring
Health checks and screening in an educational context
Recommendations vary between individuals. Your healthcare professional will advise which tests apply to you and how often.
- Discuss recurrent infections with your GP
- Attend routine vaccinations as advised
- Include inflammation markers in relevant reviews
- Follow personalised monitoring plans for autoimmune conditions
- Review any long-term immunosuppressant therapy at regular intervals
- Discuss travel plans and travel-related vaccinations in advance
Questions you may wish to discuss
Prompts for your next healthcare appointment
These are educational conversation starters — not a script. Bring the ones that feel most relevant to your situation.
- What does this result mean in my individual situation?
- Should the test be repeated, and if so when?
- Are further investigations recommended based on this result?
- Could any of my medications or supplements be influencing the result?
- Are lifestyle changes likely to help, and which ones would you prioritise?
- How does this result fit with my symptoms and medical history?
- What would prompt a change of plan or a specialist referral?
Frequently asked questions
Common educational questions
Q.What does a raised CRP mean?
A raised CRP indicates inflammation or infection somewhere in the body but does not identify the cause. It is always interpreted with symptoms and other tests.
Q.Can I boost my immune system?
The immune system is a complex balance rather than something to boost. Sleep, nutrition, exercise, stress management and vaccination all support healthy immune function.
Q.What is sepsis?
Sepsis is a life-threatening response to infection. It is a medical emergency and this module never replaces urgent professional assessment.
Q.Why are white blood cells sometimes low?
White cell counts may fall with viral infections, some medications and certain medical conditions. Interpretation depends on the wider picture.
Biomedical Scientist's insight
The science behind your result
Biomedical Scientists perform, validate and quality-check laboratory analyses before results are authorised for release. Every test is run against calibrated standards and internal controls, and reviewed for analytical accuracy. Laboratory findings are then interpreted by healthcare professionals alongside your symptoms, examination findings and medical history — which is why context matters as much as the number on the report.
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Scientific leadership
Steve Diongo
Founder & Scientific Lead · HCPC-Registered Specialist Biomedical Scientist · 15+ years NHS · 20+ years laboratory medicine
This module has been developed and reviewed by the Detectives Health Professional Team under the scientific leadership of Steve Diongo. Educational content is regularly updated using recognised laboratory standards, current scientific evidence and professional best practice.
Reviewed against NHS, NICE, CDC, WHO and USPSTF guidance.
Educational information only. Detectives Health does not diagnose disease, prescribe treatment or replace professional medical advice.
Please discuss your individual laboratory results, symptoms and health concerns with a qualified healthcare professional.