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 easily and be so serious in people with cancer.
Skin and mucous membranes
The skin is your body’s largest organ and the most important barrier against infections. It’s your first line of defense in protecting internal tissues from harmful germs. It also keeps body tissues from drying out (dehydrating). When there is a break in the skin, germs (sometimes from the air, but most often from the skin or things we touch) can enter the body and cause infection.
The mucous membranes, which form the moist, pink lining layer of the mouth, throat, nose, eyelids, urethra, vagina, and digestive (gastrointestinal) 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 invasive procedures (like putting in catheters or IVs, or getting shots) can damage the skin or mucous membranes. This makes it is easier for germs to get inside the body.
The immune system and blood cells
If germs get through the skin or mucous membranes, the job of protecting the body shifts to the immune system and some of its special cells. The 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.
How blood cells are made
Blood is made when cells in the bone marrow, called stem cells, grow into different kinds of mature cells and 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.
White blood cells help fight infection
Unlike red cells and platelets, white blood cells are part of the immune system. There are different types of white blood cells, and they each have a key role in the body’s defense against germs.
Normally, most of the white blood cells we have are neutrophils. This type of white blood cell forms a very important defense against most types of infections. In most people with cancer, having a low neutrophil count is the biggest risk factor for getting a serious infection.
Neutropenia is the medical term used to describe a shortage of neutrophils. Ask your doctor if your cancer treatment will cause neutropenia.
There are other important types of white blood cells:
- Lymphocytes (which include T-lymphocytes and B-lymphocytes)
- Monocytes
- Macrophages
Each type of white blood cell has a special job in fighting infections.
Some treatments, most often those given during bone marrow 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.
- T-lymphocytes make signaling substances called cytokines that tell other cells what to do. They also destroy cells infected by viruses.
Monocytes and macrophages have special jobs, too:
- 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.
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