How does your body normally resist infections?

Your body has many ways to protect itself from infections. It helps to understand the normal ways your body does this, and how cancer and cancer treatment change this process. This may help you better understand why infections can develop so quickly and be so serious in people with cancer.

Skin and mucous membranes

The skin is your body’s largest organ and its most 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 a partial barrier 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, radiation therapy, or surgery) and certain procedures (like putting in catheters or IVs, or getting shots) can damage the skin or mucous membranes. This makes it easier for germs to get in.

The immune system and blood cells

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 complex network of cells, signals, and organs that work together to help kill germs that cause infections. Many of these are special blood cells that travel in the blood until they find germs to attack. Others spend part of their time in the blood and the rest of their time in immune system organs.

Blood cells and how they’re made

Blood cells are made in the spongy liquid center of the bones, called bone marrow, from cells called stem cells. Stem cells grow into different kinds of mature blood cells. These mature cells are released into the blood to do their work. There are 3 major kinds of blood cells.

  • Red blood cells (RBCs or erythrocytes) carry oxygen to cells throughout the body.
  • Platelets (thrombocytes) help make clots to plug up holes that form in blood vessels from injuries such as cuts, scrapes, or bruises.
  • White blood cells (WBCs or leukocytes) help fight germs that get into the body.

See Understanding Your Lab Test Results to learn more about the tests used to measure these blood cells.

White blood cells help fight infection

White blood cells are part of the immune system. (Red cells and platelets are not.) There are different types of white blood cells, and they each have a key role in the body’s defense against germs:

  • Neutrophils
  • Lymphocytes (which include T-lymphocytes and B-lymphocytes)
  • Monocytes
  • Macrophages

Neutrophils are key infection-fighters.

Normally, most of our white blood cells are neutrophils. Neutrophils form a very important defense against most types of infection. For most people with cancer, having a low neutrophil count is the biggest risk factor for getting a serious infection. Ask your doctor if your cancer treatment will cause your neutrophil count to drop.

See Low white blood cell (neutrophil) counts and the risk of infection for more information on neutrophil counts, how the count is figured out, and what it means.

Lymphocytes can make antibodies; they mark, signal, and destroy germs.

Some treatments, most often those given during a bone marrow (stem cell) transplant, can cause a shortage of lymphocytes. B and T lymphocytes help fight viruses, but have different jobs:

  • B-lymphocytes make special proteins called antibodies that recognize and kill certain germs. They also can mark germs to be destroyed by other cells.
  • T-lymphocytes make signaling substances called cytokines that tell other cells what to do. They also destroy cells infected by viruses.

Monocytes and macrophages help recognize invaders, and kill fungi and parasites.

  • They help lymphocytes recognize germs.
  • They can surround and digest germs that have been coated by antibodies (the proteins made by B-lymphocytes).
  • They help fight bacteria, fungi, and parasites.

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.

Last Medical Review: February 16, 2015 Last Revised: February 25, 2015