Chemotherapy and infection risk: a weakened immune system

The NHS describes chemotherapy as a common cancer treatment that uses medicine to kill cancer cells. It is also sometimes used for conditions caused by the immune system attacking the body, which are autoimmune conditions. The medicines act on cells that divide quickly, and that includes some healthy cells as well as cancer cells.

Cancer Research UK explains that chemotherapy drugs often stop the bone marrow from making enough white blood cells. White blood cells are part of the body's defence against infection, so when the numbers are low, bacteria can multiply quickly in the blood and the body may not have enough white cells to fight them.

That is why the NHS lists a weakened immune system as one of the common side effects. In practice this means being immunocompromised for a limited period, rather than permanently, although how long it lasts depends on the treatment you are having.

Cancer Research UK is direct about the risk: with low white blood cells, a minor infection can become life threatening within hours. It says that all possible infections must be treated urgently and that you might need antibiotics by injection or through a drip.

This is a very different situation from catching a cold when your immune system is working normally, which is why advice for people having chemotherapy is to report signs of infection straight away rather than wait to see if they settle.

The NHS says to call your care team now if you have a high temperature, which can show up as feeling hot, cold, sweaty or shivery or the skin on your back or chest feeling hotter than usual. Cancer Research UK lists a temperature of 37.5 degrees or higher, or below 36 degrees, among the signs of infection to report urgently. Other signs it lists include:

• Feeling generally unwell, for example not being able to get out of bed.

• Flu-like symptoms such as feeling cold and shivery, headaches and aching muscles.

• A sore throat or sore mouth, or coughing up green phlegm.

• Passing urine more often, or cloudy or foul-smelling urine.

• Diarrhoea, which it describes as 4 or more loose, watery bowel movements in 24 hours.

• Redness, heat, swelling, pain or discharge at a wound or at the site of a central line or PICC line.

• A fast heartbeat, or feeling dizzy or faint.

Cancer Research UK says to call 999 or go to A&E immediately if you have possible signs of sepsis: slurred speech or confusion, extreme shivering or muscle pain, passing no urine in a day, severe breathlessness, skin that is mottled or discoloured, or a feeling that you are going to die.

The NHS also says that throbbing or cramping pain, swelling and warmth in a leg or arm, or sudden breathlessness with sharp chest pain and a cough or coughing up blood, may be signs of an infection or a blood clot that needs to be treated quickly. Use your care team's number first where you can, and call NHS 111 if you are not sure how to contact them.

The NHS says chemotherapy is usually given in cycles over several months, with a break of days or weeks between treatments to let the body recover. Before or at each treatment your care team may arrange a blood test to check whether treatment is affecting your blood cells, and ask about side effects you are having.

Most side effects, according to Cancer Research UK, are short term, begin to improve once treatment has finished, and can often be eased with medicines. Tell your care team about any side effects you get, because the NHS says they may be able to give you treatments to ease them and advice about what you can do.

Ask your own team for practical guidance specific to your regimen, such as how to take your temperature at home and what number to call out of hours. The sources used here do not give a one-size-fits-all list of precautions, and what is appropriate depends on your treatment.

Chemotherapy is one cause of lowered immunity. The NHS notes that chemotherapy is also sometimes used to treat autoimmune conditions, and that immunosuppressant medicines are used for severe lupus to calm or control the immune system. If you take any such treatment, ask your specialist team what signs of infection to report and how quickly.