How do you check your immune system in a blood test?

You can check your immune system through a combination of blood tests that assess your white blood cells, inflammation levels and the nutrients your body needs for healthy immune function. Tests like a Full Blood Count (FBC), CRP and ESR are commonly used because they show how active your immune system is and whether it’s responding to infection, inflammation or stress.

This guide explains what each test measures, what your results mean and how these markers fit together to give you a clearer picture of your immune health.

What can a blood test reveal about your immune system?

Your immune system is complex, but blood tests make it easier to understand what’s going on behind the scenes. The blood carries your infection-fighting cells, inflammatory proteins and essential nutrients, so analysing it helps identify early signs of imbalance.

A blood test can show:

• Whether your white blood cells are at healthy levels

• If your immune system is responding to infection or stress

• Whether long-term inflammation may be affecting your health

• If nutrient deficiencies could be weakening your immune defences

Rather than looking at one marker alone, doctors usually review these results together. This gives a more complete picture of how well your immune system is functioning day to day.

Using a Full Blood Count (FBC) to assess immune health

A Full Blood Count is the most widely used test for assessing immune function. It measures several types of white blood cells (WBCs), each responsible for different parts of the immune response.

What an FBC includes

Your FBC will typically measure:

• Neutrophils, which respond quickly to bacterial infections

• Lymphocytes, including T cells, B cells and natural killer cells

• Monocytes, which break down harmful bacteria and dead tissue

• Eosinophils, which play a role in allergies and parasite protection

• Basophils, linked with allergic reactions and inflammation

What your results may indicate

A higher-than-normal number of white blood cells may suggest your body is fighting an infection, dealing with inflammation or reacting to physical or emotional stress. A low count can sometimes point to a weakened immune response, especially if lymphocytes are regularly below the expected range.

Imbalances in specific types of white blood cells can offer extra clues. For example:

• High eosinophils may be linked to allergies

• Low lymphocytes can occur with certain viral infections

• Persistent changes across several white blood cell types may require further investigation

Because the full blood count is such a useful overview, it’s often the starting point when checking immune health.

CRP: Understanding inflammation and immune activity

C-reactive protein (CRP) is a protein that rises when inflammation is present. Your immune system uses inflammation to protect you, so CRP is an effective way to see whether your body is responding to something, even when you feel otherwise well.

Why CRP matters

A raised CRP can indicate:

• A current infection

• Injury or tissue damage

• Chronic low-grade inflammation

• Conditions linked to long-term inflammation

• Lifestyle factors such as smoking, stress or lack of sleep

A low CRP usually suggests low levels of active inflammation. If your CRP is slightly raised, it doesn’t necessarily mean something is wrong, it simply shows your immune system is active. Doctors often look at trends over time to understand the bigger picture.

ESR: A marker of long-term inflammation

The erythrocyte sedimentation rate (ESR) is a test that measures how quickly red blood cells settle at the bottom of a test tube. When inflammation is present, certain proteins increase, causing cells to fall more quickly.

What ESR can show

ESR is particularly useful for spotting ongoing or persistent inflammation. It changes more slowly than CRP, so it’s often used alongside it to help identify whether inflammation is recent or long-term.

A raised ESR doesn’t diagnose a specific condition, but it helps healthcare professionals understand the overall pattern of immune activity. A normal ESR, together with a normal CRP, usually indicates low inflammation.

Nutrients that support your immune system

Some blood tests assess the nutrients your immune system relies on. While these tests don’t measure immune activity, they show whether your body has the building blocks it needs to function well.

Common markers include:

• Vitamin D, which plays a key role in immune regulation

• Vitamin B12, which supports healthy cell production

• Folate, needed for tissue repair

• Iron, essential for healthy immune response and energy

Low levels of these nutrients can make you more susceptible to illness or fatigue. If your immune-related markers look imbalanced, checking your vitamin and mineral levels can help explain why.

Can a blood test show a weakened immune system?

Blood tests can highlight potential signs of immune weakness, especially when results show consistent patterns.

Results may suggest immune strain when you have:

• Low total white blood cells

• Low lymphocytes over time

• High inflammatory markers without an obvious reason

• Nutrient deficiencies that affect immune function

These findings don’t act as a diagnosis on their own, but they provide useful insight into whether your immune system may need further attention or monitoring.

What can affect immune-related blood test results?

Your immune system reacts to what’s happening in your daily life, so blood test results can shift from week to week. Short-term and long-term factors can both influence readings, including:

• Stress levels

• Poor sleep

• Smoking or high alcohol intake

• Recent infections

• Long-term inflammatory conditions

• Being run down or fatigued

• Poor diet or low nutrient intake

Your results offer a snapshot, which is why many people track their immune and inflammation levels over time rather than relying on a single test.

How often should you check your immune system?

Immune health is often assessed during routine health checks rather than on a fixed schedule. If you experience frequent infections, persistent fatigue or signs of inflammation, a blood test can help identify potential issues and guide next steps.

Regular monitoring can also be useful if:

• You're recovering from illness

• You're focusing on improving lifestyle habits

• You have long-term health conditions

• You want to track inflammation changes over time

Like all health tests, the right frequency depends on your health needs and personal goals.

FAQs

Can a blood test tell if my immune system is weak?

Yes. Consistently low white blood cells, low lymphocytes or raised inflammation markers can indicate your immune system may be under strain.

Does CRP show immune problems?

CRP shows inflammation. High levels often mean your immune system is responding to something, such as infection, injury or long-term inflammation.

Do lifestyle factors affect immune markers?

Absolutely. Stress, sleep, smoking, alcohol and poor diet can all influence immune-related blood test results.

Do I need to fast before an immune system blood test?

Most immune-related tests don't require fasting. Your test provider will let you know if any preparation is needed.

Understanding your immune health with a full body check

If you want a clearer picture of your immune health, a blood test can help you understand how your immune system is functioning and whether inflammation or nutrient levels are affecting your wellbeing.

Bluecrest's full body health checks include key immune and inflammation markers, along with a wide range of insights into your overall health. They're an accessible way to assess your immune system and track changes over time.

BOOK NOW

Need help? Call free on to speak to one of our team.