People with cancer may have a higher risk of infection because of changes in the immune system that control their body’s defense systems. Cancer and cancer treatments can affect the immune system and other body systems in different ways. People with cancer might be more likely to get infections because of:
Your cancer care team will talk to you about any increased risk for infection you may have, and what can be done to help prevent infection. If the risk is due to cancer treatment, it is usually temporary because the immune system recovers after a period of time, but this depends on your situation. You can learn more in Watching for and Preventing Infections in People With Cancer.
If you have questions about whether you need to take special precautions to prevent infections, it is best to discuss your risk of getting an infection with a doctor who understands your situation and medical history.
Your body has many ways to protect itself from infections. It helps to understand how your body normally does this, and how cancer and cancer treatment can change this process. This may help you better understand why infections can develop so quickly and be so serious in people with cancer.
The skin is your body’s largest organ and an important barrier against infections. It’s your first line of defense in protecting internal tissues from harmful germs. When there’s a break in your skin, it’s easier for germs to get into your body and cause infection.
Mucous membranes, which form the moist, pink lining layer of the mouth, throat, nose, eyelids, urethra, vagina, and digestive system, also act as partial barriers against infection. These membranes normally help protect us from germs in the air we breathe, our environment, and in our food and drink. Cancer treatments (such as chemotherapy, targeted therapy, immunotherapy, radiation therapy, or surgery) and certain procedures (like putting in catheters or IVs, or getting shots) can injure cells in the skin or cause damage to the skin or mucous membranes. This makes it easier for germs to enter the body.
If germs get through the skin or mucous membranes, the job of protecting the body shifts to your immune system. Your immune system is a group of cells, tissues, and organs that work together to help find and attack germs that invade the body and cause infections.
White blood cells, a part of the immune system, are the main type of cell responsible for protecting the body against infections. There are different types of white blood cells, and they each have a role in defending the body against infections. Normally, most of our white blood cells are neutrophils. Neutrophils are key infection-fighters and form an important defense against most types of infections. The other types of white blood cells (lymphocyte, monocytes, and macrophages) also help fight infections.
Some types of cancer can change the way the immune system blood cells work. For instance, lymphomas (Hodgkin and non-Hodgkin), multiple myeloma, and most types of leukemia start in immune system blood cells. Other types of cancer can also affect the immune system and its cells. They can change the immune system cells so that cells that once protected your body begin to interfere with the normal way your immune system works. Cancer cells can get into the bone marrow cells where blood cells are made. The cancer cells then compete with the normal bone marrow cells for space and nutrients. If too many normal bone marrow cells are destroyed or pushed out of the bone marrow, the few cells that are left won't be able to make enough white blood cells (WBCs) to help the body fight infection.
Cancer can also damage other parts of the immune system. A tumor that grows on the skin or in mucous membranes can break natural barriers and allow germs to get in. Tumors that are large might reduce blood flow to normal tissues by pressing on them or their blood supply. Tumors in the lungs may block normal mucus drainage, which can lead to infections. And, other types of tissues that have been damaged by cancer can be more prone to infections.
Certain cancer treatments can interfere with the way the immune system works. The damage can be short- or long-term. For example, if a person with cancer has their spleen removed due to cancer, this causes long-term damage because the spleen is part of the immune system. On the other hand, radiation therapy, immunotherapy, and chemotherapy, either alone or in combination can lead to short-term (temporary) immune system damage because they affect immune system blood cells for a fairly short period of time. A bone marrow or stem cell transplant uses very high-dose treatments to kill cancer cells that also damage immune system cells for weeks to months.
Any type of major surgery can weaken the immune system. Anesthesia (the drugs used to make the patient sleep) may play a role. It might take from 10 days to many months for the immune system to recover completely. Surgery also breaks the skin and can damage mucous membranes and tissue under the skin, causing it to be exposed to germs. The wound caused by surgery (the incision) is a common place for infection. Because surgery is often used to diagnose, stage, or treat people with cancer, it’s important to know that surgery can increase the risk of certain infections. Things that raise the risk of infection after surgery include:
People with cancer may get antibiotics before and for a short time after having surgery to help protect them from infection.
Chemotherapy (often called chemo) is the most common cause of a weakened immune system in people getting cancer treatment. Chemotherapy can cause neutropenia (a decrease in the number of neutrophils, a type of white blood cell, in your blood). This means your body may not be able to fight infections as well as it should. The effects on the immune system depend on many things, including:
Some drugs affect the bone marrow and immune system more than others. After treatment ends, your blood cell counts usually go back to normal over time.
Radiation therapy can also cause low white blood cell counts, which increases the risk for infections.
Factors in how radiation therapy affects the immune system, include:
Total body irradiation or TBI (where a person’s entire body is treated with radiation) is the only type of radiation likely to cause very low white blood cell counts. This type of radiation may be used during a bone marrow or stem cell transplant. Radiation is most often given to just one part of the body, so the whole immune system isn’t damaged by it. Still, depending on the dose and the part of the body being treated with radiation, the skin or mucous membranes may be damaged, so you’re less able to keep germs out. Today, radiation treatments are most often given over many sessions rather than in one large dose. This helps decrease the amount of skin and tissue damage, immune suppression, and the risk of infections.
Some types of targeted therapy can affect how the immune system works. They target a certain part of a cancer cell or a certain protein or enzyme that is on the surface of a cancer cell. Finding these targets helps the immune system see the cancer cells easier so it can attack them.
Immunotherapy is used in certain types of cancer to help the immune system recognize and attack cancer cells. This can be done by giving treatments that help your own immune system work harder or smarter, or by giving you man-made immune system proteins or altered cells that are trained to find and attack cancer cells. Immunotherapy is sometimes used by itself to treat cancer, or used along with or after another type of treatment. These treatments help the body have better immune reactions against cancer cells, but sometimes they change the way the immune system works. Because of this, people who get immunotherapy may be at risk for having a weaker immune system and getting infections.
Stem cell transplant (SCT) is the term used to include bone marrow transplant (BMT), peripheral blood stem cell transplant (PBSCT), and umbilical cord blood stem cell transplant (UCBSCT). Stem cell transplants are used to replace bone marrow cells that have been destroyed by cancer or by the chemo and/or radiation used to treat the cancer. These transplants allow doctors to use very high doses of chemo and/or total body irradiation (TBI) to try to kill all the cancer cells in the body.
In the process of killing the cancer cells, the blood-forming stem cells of the patient’s normal bone marrow are also killed. Because of this, stem cells (either from the blood or bone marrow) are removed from the patient and saved before the high-dose chemo is given. Or, stem cells may be taken from a donor or banked umbilical cord blood. Once the cancer cells are killed, the saved or donated stem cells are given to the patient so that blood cells can be made and the immune system rebuilt. High-dose chemo used with TBI causes more severe immune weakness that lasts for a longer time. It can also damage the skin and mucous membranes and make them less able to keep germs out of the body. This increases the risk of infection.
All cells need nutrients to grow and work. Lack of vitamins, minerals, calories, and protein can weaken your immune system and make it less able to find and destroy germs. This means people who are poorly nourished (malnourished) are more likely to develop infections. People who are malnourished either do not take in enough calories and nutrients, or the body can’t use the food it takes in. Either way, it can weaken your immune system.
People with cancer often need extra calories and protein to support their immune system cells and other tissues. For example, recovery from surgery increases the body’s need for nutrients.
People with cancer might be poorly nourished for many reasons:
People with cancer often need help from dietitians or doctors to get enough calories and nutrients. Dietary supplements, tube feedings, or even intravenous (IV, through a vein) feedings may be needed to help in some cases.
It's important to know that avoiding or eating certain kinds of foods will not affect white blood cell counts (one reason your immune function may not be normal). However, your doctor might have you meet with a dietitian to plan what you should eat and to get help managing eating problems. Nutrition counseling should include the importance of getting enough calories, protein, and vitamins. This is tailored to each person’s food intake and nutrition problems.
Learn more about what to eat during cancer treatment in Nutrition for the Person With Cancer.
Certain cancer treatments (such as chemotherapy, radiation therapy, surgery, stem cell or bone marrow transplant, or steroids) or the cancer itself can suppress or weaken the immune system. These treatments can lower the number of white blood cells (WBCs) and other immune system cells. Treatment can also cause these cells to not work as well as they should. This is called immunosuppression. It’s much easier to get an infection when there aren’t enough WBCs to destroy germs, especially the type of WBCs called neutrophils.
Neutrophils are a very important defense against most types of infection. When looking at your risk of getting an infection, doctors look at the number of neutrophils you have. A low neutrophil count is called neutropenia . The doctor may say you are neutropenic.
The American Cancer Society medical and editorial content team
Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.
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Last Revised: March 13, 2020
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