The immune system and cancer

What the immune system does

The immune system protects the body against illness and infection that bacteria, viruses, fungi or parasites can cause. It is a collection of reactions and responses that the body makes to damaged cells or infection. So it is sometimes called the immune response.

The immune system is important to people with cancer because:

• cancer can weaken the immune system

• cancer treatments might weaken the immune system

• the immune system may help to fight cancer

Cancer and treatments may weaken immunity

Cancer can weaken the immune system by spreading into the bone marrow. The bone marrow makes blood cells that help to fight infection. This happens most often in leukaemia or lymphoma, but it can happen with other cancers too. The cancer can stop the bone marrow from making so many blood cells.

Certain cancer treatments can temporarily weaken the immune system. This is because they can cause a drop in the number of white blood cells made in the bone marrow. Cancer treatments that are more likely to weaken the immune system are:

• chemotherapy

• targeted cancer drugs

• radiotherapy

• high dose of steroids

Find out about the different types of cancer treatments

The immune system can help to fight cancer

Some cells of the immune system can recognise cancer cells as abnormal and kill them. But this may not be enough to get rid of a cancer altogether.

Some treatments aim to use the immune system to fight cancer.

There are 2 main parts of the immune system:

• the protection we have from birth (in built immune protection)

• the protection we develop after having certain diseases (acquired immunity)

In built immune protection

This is also called innate immunity. These mechanisms are always ready and prepared to defend the body from infection. They can act immediately (or very quickly). This in built protection comes from:

• a barrier formed by the skin around the body

• the inner linings of the gut and lungs, which produce mucus and trap invading bacteria

• hairs that move the mucus and trapped bacteria out of the lungs

• stomach acid, which kills bacteria

• helpful bacteria growing in the bowel, which prevent other bacteria from taking over

• urine flow, which flushes bacteria out of the bladder and urethra

• white blood cells called neutrophils, which can find and kill bacteria

Different things can overcome and damage these natural protection mechanisms. For example:

• something may break the skin barrier, such as having a drip in your arm or a wound from surgery

• a catheter into your bladder can become a route for bacteria to get inside the bladder and cause infection

• anti acid medicines for heartburn may neutralise the stomach acid that kills bacteria

Certain cancer treatments can also overcome these protection mechanisms. Chemotherapy can temporarily reduce the number of neutrophils in the body, making it harder for you to fight infections. Radiotherapy to the lung can damage the hairs and mucus producing cells that help to remove bacteria.

Neutrophils

Neutrophils are a type of white blood cell that are very important for fighting infection. They can:

• move to areas of infection in the body

• stick to the invading bacteria, viruses or fungi

• swallow up the bacteria, viruses or fungi and kill them with chemicals

When you don't have enough neutrophils in your blood, doctors may say that you are neutropaenic.

Chemotherapy, targeted cancer drugs and some radiotherapy treatments can lower the number of neutrophils in the blood. So you might get more bacterial or fungal infections after these treatments.

It is important for you to know the following when having cancer treatment:

• infections can become serious very quickly in people with low neutrophil counts

• antibiotics could save your life so if you get a fever or feel ill phone your advice line or go to hospital straight away

• you might need to take antibiotics to help prevent severe infection if your blood counts are low

It is more usual to become ill from bugs you carry around with you than from catching someone else's. This means that you shouldn't have to avoid contact with your family, friends or children after treatment.

You can ask your doctor or nurse what precautions you should take against infection.

Acquired immunity

This is immune protection that the body learns after having certain diseases. The body learns to recognise each different kind of bacteria, fungus or virus it meets for the first time. So the next time the same bug invades the body it is easier for the immune system to fight it. This is why you usually only get some infectious diseases such as measles or chicken pox once.

Vaccination works by using this type of immunity. A vaccine contains a small amount of protein from a disease. This is not harmful but it allows the immune system to recognise the disease if it meets it again. The immune response can then stop you getting the disease.

Some vaccines use small amounts of the live bacteria or virus. These are live attenuated vaccines. It means that scientists have changed the virus or bacteria so that it stimulates the immune system to make antibodies. A live vaccine won't cause an infection.

Other types of vaccine use killed bacteria or viruses, or parts of proteins that bacteria and viruses produce.

B cells and T cells

Lymphocytes are a type of white blood cells involved in the acquired immune response. There are 2 main types of lymphocytes:

• B cells

• T cells

The bone marrow produces all blood cells, including B and T lymphocytes. Like the other blood cells, they have to fully mature before they can help in the immune response.

B cells mature in the bone marrow. But T cells mature in the thymus gland. Once they are mature, the B and T cells travel to the spleen and lymph nodes ready to fight infection.

You can read about the thymus, spleen and lymph nodes on our page about the lymphatic system and cancer.

What B cells do

B cells react against invading bacteria or viruses by making proteins called antibodies. Your body makes a different antibody for each different type of germ (bug). The antibody locks onto the surface of the invading bacteria or virus. This marks the invader so that the body knows it is dangerous and needs to be killed. Antibodies can also find and kill damaged cells.

The B cells are part of the memory of the immune system. The next time the same germ tries to invade the B cells that make the right antibody are ready for it. They are able to make their antibody very quickly.

How antibodies work

Antibodies have 2 ends. One end sticks to proteins on the outside of white blood cells. The other end sticks to the germ or damaged cell and helps to kill it. The end of the antibody that sticks to the white blood cell is always the same. Scientists call this the constant end.

The end of the antibody that recognises germs and damaged cells varies, depending on the cell it needs to recognise. So it is called the variable end. Each B cell makes antibodies with a different variable end from other B cells.

Cancer cells are not normal cells. So some antibodies with variable ends recognise cancer cells and stick to them.

What T cells do

There are different kinds of T cells called:

• helper T cells

• killer T cells

The helper T cells stimulate the B cells to make antibodies and help killer cells develop.

Killer T cells kill the body's own cells that have been invaded by the viruses or bacteria. This prevents the germ from reproducing in the cell and then infecting other cells.

Cancer treatments that use the immune system

Some cancer treatments use parts of the immune system to help treat cancer.

Immunotherapy

Immunotherapy is a treatment for some types of cancer. It uses the immune system to find and kill cancer cells.

They are helpful in cancer treatment because cancer cells are different from normal cells. And the immune system can recognise and kill abnormal cells.

In the laboratory scientists can produce different chemicals that are part of the immune response. So, they can make different types of immunotherapies such as:

• monoclonal antibodies (MABs), which recognise and attack certain proteins on the surface of cancer cells

• vaccines to help the immune system to recognise and attack cancer

• cytokines to help to boost the immune system

• CAR T-cell therapy (also called adoptive cell transfer) to change the genes in a person's white blood cells

Read more about immunotherapy