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Immunity and infection prevention

Immune memory

Some illnesses we catch only once in a lifetime, and a vaccine can protect for years. The reason is immune memory: the immune system “remembers” a germ it has met before. This topic explains how memory forms, how long it lasts and how leukemia treatment, especially a transplant, can change it.

Last updated: ~5 min read
Small, calmly glowing cells on a dark navy background, each holding its own faint gold shape, lined up like shelves in an archive.
Figure 1.Concept: memory cells keep “portraits” of past germs.

01Short answer

Immune memory is the defining feature of adaptive immunity. At a first encounter with a germ, it takes several days to find and multiply the matching B and T cells. After the fight, some of these cells remain for a long time as memory cells, and some plasma cells keep making antibodies for years. When the same germ returns, the response is faster and stronger, and the illness either goes unnoticed or is mild. Vaccines exist to create this memory without the illness. How long memory lasts depends on the germ: sometimes for life, sometimes booster doses are needed. Leukemia itself, chemotherapy, medicines that target B cells and especially a stem cell transplant can weaken or “erase” memory. This is why doctors sometimes recommend repeating vaccines after treatment.

02Simple explanation

Think of the immune system as a student. At its first meeting with a germ, it has to learn to recognise the enemy. This takes several days, and during that time a person may fall ill.

Once the fight is over, most of the fighting cells die, but some become memory cells. They keep a “portrait” of the enemy and stand guard in the body for a long time.

When the same germ comes back a second time, memory cells recognise it at once and the response is faster. Often the person does not even notice they were ill.

A vaccine shows the immune system a killed or weakened germ, or a part of one. In this way memory forms without the illness.

The link with leukemia: intensive treatment and especially a transplant can weaken memory. So after treatment, some vaccines may need to be repeated, and your hematology team plans this.

03Scientific explanation

Memory cells: according to InformedHealth, after an infection some helper T cells become memory T cells, and some activated B cells become memory B cells. The first response can take several days, but the next time the body reacts straight away, and a second infection often goes unnoticed or is milder.

Three types of immunity: according to the NCI dictionary, immunity can be innate, adaptive or passive. Adaptive immunity develops after infection or vaccination and sometimes lasts a lifetime. Passive immunity means receiving ready-made antibodies: it protects immediately but lasts only a few weeks or months.

How long does memory last? In a study by Amanna and colleagues with follow-up of up to 26 years, antibody half-lives were estimated at more than 200 years for measles and mumps, about 50 years for the chickenpox virus, 11 years for tetanus and 19 years for diphtheria. This helps explain why some vaccines need booster doses.

Innate “memory”: according to a review by Farber and colleagues, immune memory is classically seen as a feature of adaptive immunity, but recent research suggests that innate immune cells can also show memory-like behaviour. This area is still being actively studied.

04What happens in the body?

First and second responses

At a first encounter, antibodies appear after several days, and most of them are IgM. At a second encounter, memory B cells quickly turn into plasma cells and make more, better-fitting IgG. That is why the second response is both faster and stronger.

Two waves of light on a dark navy background: one small and slowly rising, the second taller and rising quickly in turquoise and gold.
Figure 2.The second response is faster and stronger than the first.

How do leukemia and treatment affect memory?

According to the American Cancer Society, some blood cancers, chemotherapy, immunotherapy and transplants weaken the immune system. According to the CDC, memory from childhood vaccines usually remains after chemotherapy, but some vaccines may need repeating after treatment for ALL. Medicines that target B cells can delay the formation of new memory for months.

After a transplant: learning again

According to the ECIL 7 guidelines, most people who have a stem cell transplant lose their immunity to various germs within the first months, regardless of earlier vaccinations. The new immune system gradually responds better to vaccines. That is why vaccinations are restarted according to the team’s plan.

Small glowing cells rising from young translucent bone marrow tissue on a dark navy background, bathed in soft dawn light.
Figure 3.After a transplant, the new immune system relearns germs.

Sleeping viruses and memory

After chickenpox, the virus stays in the body in a “sleeping” state, and immune memory keeps it under control. According to MedlinePlus, people with weakened immune systems, including those with some cancers or taking steroids, are at higher risk of this virus waking up as shingles (herpes zoster).

How can you protect your memory?

Keep your vaccination records and show them to your team. Where possible, discuss any vaccines you need before treatment starts. Vaccinating family members also protects you. Live vaccines are usually not given to people with weakened immunity; your team decides which vaccine is needed and when.

05How does memory form?

A simplified sequence.

First encounter — The immune system meets a germ or a vaccine.1First encounterThe immune system meets a germ or a vaccine.
Learning — Matching B and T cells multiply over several days.2LearningMatching B and T cells multiply over several days.
Memory — Some cells become long-lived memory cells.3MemorySome cells become long-lived memory cells.
Fast response — At the next encounter, defence starts at once.4Fast responseAt the next encounter, defence starts at once.
SpeedThe second response comes sooner.
StrengthMore and better-fitting antibodies.
DurationSometimes lifelong, sometimes a booster is needed.

06What affects immune memory?

SituationEffect on memoryWhat is usually done
Healthy immune systemMemory lasts for years, sometimes for lifeScheduled vaccines and boosters
ChemotherapyEarlier memory usually remains; new responses may be weakVaccines limited during treatment; the doctor reassesses afterwards
Medicines that target B cellsNew antibody responses are weakened for monthsVaccine timing is planned with the team
Stem cell transplantMost earlier memory is lostVaccinations are restarted according to a plan
Ready-made antibodies (immunoglobulin)Do not create your own memory; protect temporarilyAs prescribed by your doctor

* This table is for general information only. The exact timing and type of vaccines are decided only by your hematology team.

07Key terms

Immune memory
The immune system’s ability to recognise a germ it has met before and respond faster.
Memory B cell
A B cell that remembers a germ and quickly switches to making antibodies when it meets it again.
Memory T cell
A T cell that remembers a germ and quickly starts the response.
Adaptive immunity
Immunity with memory that develops after infection or vaccination.
Passive immunity
Temporary protection from receiving ready-made antibodies.
Vaccine
A way of introducing a germ to the immune system without causing illness.
Booster dose
An extra vaccine dose given to refresh fading memory.
Live vaccine
A vaccine containing a weakened live germ.
Half-life
The time it takes for an antibody level to fall by half.
Shingles (herpes zoster)
The reawakening of the sleeping chickenpox virus.

08Key points

  • Immune memory is the defining feature of adaptive immunity.
  • The first response takes several days; the second is faster and stronger.
  • Memory is held by memory B and T cells and long-lived plasma cells.
  • Vaccines create memory without the illness.
  • How long memory lasts depends on the germ.
  • A transplant can erase most earlier memory.
  • Medicines that target B cells can delay new responses.
  • Plan your vaccinations only with your team.

09Frequently asked questions

Do my childhood vaccines still work?

Most protect for years, but some, such as tetanus, need boosters. After leukemia treatment, your doctor will assess which vaccines need updating.

Why do I need my vaccines again after a transplant?

According to ECIL 7, most earlier immunity is lost after a transplant. The new immune system has to “relearn” germs, and vaccines do this safely.

Can I have vaccines during treatment?

Some vaccines, such as the flu vaccine, may be recommended during treatment too, but the response may be weaker. Live vaccines are usually not given. Your team decides; we cover this in more detail in the vaccines topic.

Do antibody tests show my immune memory?

Partly. Antibody levels reflect part of the memory, but memory cells and T-cell responses do not show up on a standard test. Sometimes a doctor orders antibody tests to check the response to a vaccine.

Does an immunoglobulin infusion protect me like a vaccine?

No. According to the NCI, ready-made antibodies give passive immunity: they protect immediately but only for a few weeks or months, and they do not create your own memory.

What should my family do?

According to the American Cancer Society, vaccinating family members and close contacts is important to protect a person with weakened immunity. They should discuss which vaccines suit them with their own doctors.

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. Does treatment wipe out the protection from earlier vaccines?

  2. Will I keep immunity to infections I had in childhood?

  3. Will I need to be vaccinated again?

  4. How can the recovery of immune memory be checked?

My questions

12Sources

  1. InformedHealth.org (IQWiG). The innate and adaptive immune systems
  2. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: immunity
  3. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: adaptive immunity
  4. Journal of Allergy and Clinical Immunology, 2010. Chaplin DD. Overview of the immune response
  5. Nature Reviews Immunology, 2016. Farber DL et al. Immunological memory: lessons from the past and a look to the future
  6. New England Journal of Medicine, 2007. Amanna IJ, Carlson NE, Slifka MK. Duration of humoral immunity to common viral and vaccine antigens
  7. World Health Organization (WHO). Vaccines and immunization: What is vaccination?
  8. Centers for Disease Control and Prevention (CDC). Altered Immunocompetence — General Best Practice Guidelines for Immunization
  9. Clinical Infectious Diseases (IDSA), 2014. 2013 IDSA Clinical Practice Guideline for Vaccination of the Immunocompromised Host
  10. The Lancet Infectious Diseases (ECIL 7), 2019. Vaccination of haemopoietic stem cell transplant recipients: guidelines of the 2017 European Conference on Infections in Leukaemia (ECIL 7)
  11. The Lancet Infectious Diseases (ECIL 7), 2019. Vaccination of patients with haematological malignancies who did not have transplantations: guidelines from the 2017 European Conference on Infections in Leukaemia (ECIL 7)
  12. American Cancer Society. Vaccinations During Cancer Treatment
  13. MedlinePlus (U.S. National Library of Medicine). Shingles
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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