Hemoglobin, the protein in red blood cells that oxygenates the blood, provides the body with energy, strength, and stamina. When there isn't enough oxygen in the blood, he'll feel tired, weak, and sometimes faint or dizzy. He may have trouble catching his breath or feel pain in his chest as he tries to breathe. Extreme fatigue can bring with it memory or concentration problems, so he might seem confused or act like he's not thinking straight. For more information, see Understanding Low Red Blood Counts.

Platelets control clotting and stop bleeding, so when his platelets are low, he bruises easily and his body has trouble stopping the bleeding from even a small cut or scratch. (Women who are still having periods may have unusually heavy flow.) Nosebleeds and bleeding gums after brushing teeth can become an ongoing problem. Some people also get a rash that looks like little pinprick-sized red or purple dots, or they experience aching in their joints or muscles. Headaches are common too. For more information, see Understanding Low Platelets.

The technical name for low white blood cell count is neutropenia, from the word neutrophil, and it means that his immune system is depressed. Another term you may hear is immunocompromised. Without regular immune function, his body can't fight off germs and infections as it normally would. Watch closely for signs of fever, sore throat or cough, or chills and shaking. Keep an eye out for other signs of infection such as an injury that gets red and pus-filled or doesn't heal. Another sign is diarrhea or loose stools for more than two days in a row. On the opposite extreme, an overly high white blood count is a sign his body is fighting an infection, although those fighting leukemia also can have elevated white counts.