Red blood cells are a key life force of the body, playing an essential role in carrying oxygen from the lungs to the cells. That's why when someone's red blood count drops, he really feels it -- and shows it. He may be pale, tired, weak, and have trouble catching his breath.

Why does cancer cause red blood cell (RBC) counts to drop?
-- In most cases, it's not the cancer itself but cancer treatment that leads to a shortage of red blood cells, also called anemia. Chemotherapy often damages the bone marrow that produces red and white blood cells and platelets, a condition called bone marrow suppression, or myelosuppression. As red blood cells die out, which they do naturally every 120 days or so, the body isn't able to replace them, and the red blood count drops.

-- Radiation therapy can also damage red blood cell production, particularly if the radiation targets areas such as the pelvis, where bones have more marrow and higher blood cell-generation activity.

-- Blood loss, either from surgery or from particular cancers, can also cause or exacerbate anemia. Colorectal cancers, for example, often cause blood loss as blood leaks from the intestines. Cancers that affect the blood and bone marrow, particularly lymphoma and Leukemia, can also damage the production of red blood cells.

What are the symptoms of low red blood count?
The typical signs of anemia are extreme fatigue and shortness of breath, but a low red blood count can cause a host of different symptoms. A patient's skin may be pale or clammy, or he may have a rapid heart rate, chest pain, or difficulty staying warm. Because low oxygen in the blood can affect the brain as well, he may feel dizzy and light-headed or have difficulty concentrating or remembering things. He may also have headaches.

Can I help prevent someone's red blood count from dropping?
There are medications called cell growth factors that can be given along with chemo or radiation to prevent red blood counts from dropping. The growth factor that boosts production of red blood cells is epoetin or darbepoetin (brand names Procrit, Epogen, Aranesp). Growth factors can take between two and eight weeks to work, so they're not a solution for dramatic red blood cell loss. There's also a risk of side effects, so a patient's doctor will decide whether to prescribe cell growth factors based on her assessment of the patient's risk of developing a low red blood count.

Get as much information as you can from the doctor about the chemo regimen of the person you're caring for and the effect you can expect it to have on his red blood count. "Some drugs reduce the red blood cell count by 25 percent immediately; some drugs take a week before the count start to drop. Every drug and regimen is different," says Terry Anders, an oncology nurse at the Zangmeister Cancer Center in Columbus, Ohio. Because side effects include fluid retention and heart palpitations -- both of which can worsen heart conditions -- the doctor will take into account whether the patient has heart problems when deciding whether to prescribe Procrit or other growth factors. Other side effects include diarrhea, vomiting, fever, and upper respiratory infection. In addition, Procrit can cause shortness of breath and edema, so if the person you're caring for already has these problems, the doctor may be conservative about prescribing it.