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Immunity and infection prevention

Fighting a microbe

You cut your finger, and a few hours later the wound is red, swollen and warm. This is a sign that your body has started fighting a germ. This topic explains, step by step, what happens from the moment a germ enters the body until recovery, and shows why this fight can look different in leukemia.

Last updated: ~5 min read
Small dark particles seeping through a thin crack between translucent layers of skin on a dark navy background, with glowing cells moving towards them.
Figure 1.Concept: a germ tries to get in through a small break in the skin.

01Short answer

Fighting a germ happens in several stages. First, the skin and mucous membranes try to keep the germ out. When a germ gets past the barrier, innate immune cells recognise it within minutes to hours and inflammation begins: blood vessels widen, and the area turns red, warms up, swells and hurts. Neutrophils and macrophages swallow germs, while cytokines call other cells for help and can cause a fever. If the germ remains, adaptive immunity, with antibodies and killer T cells, joins in over several days. After the fight, the tissue repairs itself and memory remains. In leukemia, especially when neutrophils are low, this fight can be weak and the usual signs, such as pus, redness and swelling, may not appear. That is why, during treatment, a fever is often the only sign of infection and needs urgent attention.

02Simple explanation

Entry: a germ tries to get into the body through a small cut, the nose, the mouth or the gut. The skin and mucous membranes are the first barrier.

The alarm: local immune cells quickly recognise a germ that gets in and send out “help needed” signals.

Inflammation: more blood flows to the area, so it turns red and warm. Fluid and immune cells leave the blood vessels, so it swells. Nerves are irritated, so it hurts; this is the body’s way of telling you to protect that spot.

The main forces: if the germ is not stopped, B and T lymphocytes are switched on in the lymph nodes. After several days, antibodies and killer cells target the germ precisely.

The link with leukemia: when neutrophils are low, inflammation is weak and a wound may look “quiet”, but the infection can spread quickly. A fever or feeling unwell during treatment is a reason to call urgently.

03Scientific explanation

How does inflammation happen? According to InformedHealth, during inflammation immune cells release substances such as histamine and bradykinin. These widen the small blood vessels, so the area turns red and hot; they irritate nerves, causing pain, and make it easier for immune cells to leave the vessels. Fluid passing into the tissue causes swelling.

Speed: according to InformedHealth, the innate immune system destroys bacteria that enter the skin through a small wound, for example, on the spot within a few hours. The first response of the adaptive immune system, by contrast, can take several days.

Fever: according to MedlinePlus, a fever is not a disease but a sign that the body is fighting an infection: many bacteria and viruses multiply better at normal body temperature. According to MedlinePlus, proteins that cause fever, such as interleukin-1, are part of innate immunity.

The fight during neutropenia: according to IDSA guidance, in patients with neutropenia the signs of inflammation may be weak or absent, so a fever is often the first and sometimes the only sign of infection. According to the NCI, a fever of 38 °C or higher during treatment should be reported straight away.

04What happens in the body?

What is pus?

According to InformedHealth, it is not only germs that die in the fight, but also tissue cells and immune cells. Together, their remains form a yellowish fluid called pus. Pus shows that the immune system, especially the neutrophils, is at work. When neutrophils are very low, hardly any pus may form.

An area of tissue glowing warm red and gold on a dark navy background, with many small light-filled cells leaving widened blood vessels.
Figure 2.Inflammation: widened blood vessels and immune cells rushing to help.

Why do lymph nodes swell?

Dendritic cells carry pieces of the germ to the nearest lymph node. There, matching lymphocytes multiply and the node enlarges, sometimes becoming tender. For example, during a throat infection, the nodes in the neck swell. Once the infection has passed, the nodes usually shrink.

When do antibiotics help?

According to the CDC, antibiotics act on bacteria but do not treat viral infections such as colds and flu. During treatment, if an infection is suspected, the doctor often starts antibiotics without waiting for test results. Take the medicine exactly as prescribed and do not stop it on your own.

After the fight

Once the germ is defeated, inflammation settles: fluid drains from the tissue, swelling goes down, macrophages clear away dead cells and the tissue repairs itself. Adaptive immunity keeps memory cells so that the next fight is faster.

Smooth, repaired tissue in calm turquoise light on a dark navy background, with a few small, softly glowing memory cells above it.
Figure 3.After the fight, the tissue calms down and repairs itself.

What to watch for during treatment

According to the NCI and the CDC, signs of infection during treatment can include fever, chills, cough, sore throat, diarrhoea, a rash, mouth sores, and redness or swelling where a catheter enters the body. Always keep your team’s emergency number with you and call straight away if you have a fever.

05The germ’s path and the body’s response

From a small cut to recovery (simplified).

Entry — The germ gets through a break in the skin or a mucous membrane.1EntryThe germ gets through a break in the skin or a mucous membrane.
Inflammation — Redness, warmth, swelling and pain; phagocytes swallow the germ.2InflammationRedness, warmth, swelling and pain; phagocytes swallow the germ.
Adaptive response — Lymphocytes multiply in a lymph node and antibodies appear.3Adaptive responseLymphocytes multiply in a lymph node and antibodies appear.
Recovery — The tissue repairs itself and memory remains.4RecoveryThe tissue repairs itself and memory remains.
UsuallyThe infection is stopped where it started.
During neutropeniaSigns are weak and the risk is high.
With a feverDuring treatment, call straight away.

06Signs of inflammation and what causes them

SignWhat happensDuring neutropenia
RednessBlood vessels widen and more blood flows inWeak or may be absent
WarmthIncreased blood flow warms the areaMay be weak
SwellingFluid and cells move from vessels into the tissueMay be weak
PainInflammatory substances irritate nervesOften still present
PusRemains of dead germs and cellsMay be almost absent
FeverSignalling proteins raise body temperatureOften the only sign

* This table is for general information only. During treatment, tell your team about any suspected infection.

07Key terms

Microbe (germ)
A tiny living thing such as a bacterium, virus, fungus or parasite.
Infection
When germs enter the body and multiply.
Inflammation
A response to injury or infection with redness, warmth, swelling and pain.
Histamine
An inflammatory substance that widens blood vessels.
Cytokine
A protein that carries signals between immune cells.
Fever
A body temperature higher than normal.
Pus
A yellowish fluid made of the remains of dead germs and cells.
Phagocytosis
When a cell swallows and digests a germ.
Neutropenia
A low number of neutrophils in the blood.
Antibiotic
A medicine that acts against bacteria.

08Key points

  • The skin and mucous membranes are the first barrier.
  • Innate immunity responds within hours.
  • Inflammation shows as redness, warmth, swelling and pain.
  • A fever is a sign that the body is fighting an infection.
  • Adaptive immunity joins in after several days and creates memory.
  • During neutropenia, signs of inflammation can be weak.
  • During treatment, a fever is often the only sign of infection.
  • If you have a fever, call your team straight away.

09Frequently asked questions

Should I bring a fever down straight away?

During treatment, call your team first. Fever-reducing medicines can mask an infection, and some are not suitable when platelets are low. Your team will tell you what to take.

Why can a wound look normal when my neutrophils are low?

Because redness, swelling and pus are mostly the work of immune cells. According to the IDSA, these signs can be weak during neutropenia, but the infection can still be serious.

Does every fever mean an infection?

No, a fever can sometimes be linked to medicines, blood transfusions or the disease itself. But during treatment it is treated as an infection until proven otherwise and assessed urgently.

Do antibiotics help against viruses?

According to the CDC, antibiotics do not treat viral infections. Special antiviral medicines are sometimes used against viruses; your doctor decides.

What can I do at home for mild inflammation?

Keep a small cut clean and keep an eye on it. During treatment, though, tell your team about any redness, swelling or pain, especially at a catheter site, and do not try to treat it yourself.

What is sepsis?

Sepsis is a life-threatening condition in which the body’s response to an infection damages its own organs. Chills, a very high fever, a racing heart and feeling extremely unwell need emergency help.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. How does my body fight infection if my neutrophils are low?

  2. Why might signs of infection be faint in my case?

  3. Which signs could show that an infection is starting?

  4. What should the first step be if I suspect an infection?

My questions

12Sources

  1. InformedHealth.org (IQWiG). What is an inflammation?
  2. InformedHealth.org (IQWiG). The innate and adaptive immune systems
  3. InformedHealth.org (IQWiG). How does the immune system work?
  4. MedlinePlus (U.S. National Library of Medicine). Immune response
  5. MedlinePlus (U.S. National Library of Medicine). Fever
  6. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: inflammation
  7. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: cytokine
  8. Journal of Allergy and Clinical Immunology, 2010. Chaplin DD. Overview of the immune response
  9. Allergy, Asthma & Clinical Immunology, 2018. Marshall JS et al. An introduction to immunology and immunopathology
  10. National Cancer Institute (NCI). Infection and Neutropenia during Cancer Treatment
  11. Centers for Disease Control and Prevention (CDC). Neutropenia and Risk for Infection
  12. Clinical Infectious Diseases (IDSA), 2011. Clinical practice guideline for the use of antimicrobial agents in neutropenic patients with cancer: 2010 update by the Infectious Diseases Society of America
  13. Journal of Clinical Oncology (ASCO and IDSA), 2018. Outpatient Management of Fever and Neutropenia in Adults Treated for Malignancy: ASCO and IDSA Clinical Practice Guideline Update
  14. Centers for Disease Control and Prevention (CDC). Healthy Habits: Antibiotic Do’s and Don’ts
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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