How Chemotherapy Affects the Immune System

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Chemotherapy is the cancer treatment most likely to weaken the immune system. Chemotherapy medicines target rapidly dividing cells, which cancer cells are — but so are many of the normal cells in your blood, bone marrow, mouth, intestinal tract, nose, nails, vagina, and hair. So chemotherapy affects them, too. Cancer cells are destroyed by chemotherapy because they can’t repair themselves very well. Your healthy cells typically can repair the damage from chemotherapy once treatment ends. (One notable exception is nerve cells in your hands and/or feet, which can be permanently damaged by certain chemotherapy medications — a condition known as peripheral neuropathy.)

As chemotherapy medicines damage the bone marrow, the marrow is less able to produce enough red blood cells, white blood cells, and platelets. Typically, the greatest impact is on white blood cells. When you don’t have enough white blood cells, your body is more vulnerable to infection.

Although most chemotherapy medications can have an impact on your immune system, how much of an impact depends on many factors, such as:

  • which medicines you’re taking and in what combination — having two or three at once is more likely to affect the immune system than having one
  • how much medicine is given and how often medicine is given (dosing)
  • how long treatment lasts
  • your age and overall health
  • other medical conditions you have

Some chemotherapy medicines are taken by mouth, in pill form, while others are given intravenously — through a vein in the chest, arm, or hand — at a hospital or clinic. If you’re having intravenous treatment, ask that it be given on the opposite side of the body from where you had your surgery. The injection site poses some risk of infection, and since breast cancer surgery usually removes lymph nodes, you definitely want to minimize that risk on the affected side of your body. (If you had cancer in both breasts, choose the side of the body that had less extensive surgery or fewer lymph nodes removed, if possible.)

The timing of different chemotherapy regimens varies. Typically, you would take the medication(s) for one day to several days, wait a couple of weeks to give the body time to recover, and then start the cycle again. Treatment can last for anywhere from 3 to 6 months. During that time, you would be considered to be immunocompromised — not as able to fight infection. After finishing chemotherapy treatment, it can take anywhere from about 21 to 28 days for your immune system to recover.

What you and your doctor can do about chemotherapy’s effects on the immune system

If chemotherapy is part of your treatment plan, you and your doctor should review the medications you’ll have and discuss potential effects on your immune system.

Before, during, and after chemotherapy, do your best to follow the common-sense ways to take care of your immune system, such as getting enough rest, eating a healthy diet, exercising, and reducing stress as much as you can. Some chemotherapy medicines can reduce your appetite and make you feel tired, so ask your doctor about ways to manage those side effects.

Before you start chemotherapy, your doctor should order a complete blood count (CBC) to check your baseline levels of different blood cells, including white blood cells. You’ll continue to have this blood test done periodically throughout your treatment. When your white blood cell count is lower than normal, you’re more prone to infection. Especially important is a type of white blood cell known as neutrophils, which are first responders to infection that can gobble up bacteria, fungi, and germs. Your test results will include an absolute neutrophil count, or ANC. Usually, your neutrophil levels start to drop about a week after your chemotherapy cycle begins, reach a low point in another week or so, and then slowly begin to climb again before your next cycle of treatment. Blood tests will help your doctor know if your neutrophil levels have bounced back enough in between treatments.

A normal neutrophil count is around 2,500-6,000. If yours is lower than that, and especially down to 1,000 or lower, your risk of infection is increased. If the count falls below 500, you have a condition called neutropenia, which greatly raises your risk of a serious infection.

Whatever your situation, it’s very important to follow specific steps for protecting yourself against infection and to promptly report any signs or symptoms of infection to your doctor. When your immune system is weak, an infection can worsen quickly and even turn life-threatening. If you have a fever higher than 100 and suspect infection but you can’t reach your doctor, seek emergency medical attention.

If your neutrophil levels don’t bounce back quickly enough between treatments or you develop neutropenia, your doctor may decide to:

  • delay your next round of chemotherapy, or reduce the dose
  • give antibiotics along with your treatments to prevent infection

If chemotherapy causes neutropenia accompanied by a fever, your doctor may prescribe medications called colony-stimulating factors (CSFs) or white blood cell growth factors to be given along with your remaining chemotherapy treatments. These medications can help the body produce more neutrophils and other types of white blood cells, which strengthens your ability to fight off infection. Examples include:

These are given as a series of shots in between treatment cycles. Although CSFs can reduce the risk of hospitalization due to infection, they can cause side effects such as aches in the bones, low-grade fever, and fatigue. Generally, CSFs are used in people who are on a chemotherapy regimen that more commonly causes neutropenia or for those who aren’t helped by an adjustment in the chemotherapy dose. Talk to your doctor to find out what is recommended for you.

Even after you finish treatment, it is important to follow steps for protecting yourself against infection until your immune system returns to normal.

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