When a person has leukemia, the disease affects the blood-forming tissues of the body, including the bone marrow and the lymphatic system, states Mayo Clinic. This leads to abnormalities in the blood, which typically begin with an increase in the number of white blood cells. Eventually, the abnormal white blood cells "fill up" the bone marrow, preventing it from producing other normal cells, explains the Leukemia & Lymphoma Society.
There are many different types of leukemia, but in all cases, the disease involves the production by the bone marrow of abnormal cells called "blasts." These cells divide more rapidly than normal cells, do not function properly and have a tendency to spread to other organs, such as the lymph glands, the brain and spinal cord, and the testicles, as the Leukemia & Lymphoma Society explains. As the disease progresses, these abnormal cells also interfere with the body's ability to produce red blood cells, which carry oxygen, and platelets, which control bleeding. Because the abnormal white blood cells don't fight infection like normal white cells, the person is more susceptible to infections as well.
Symptoms of leukemia include tiredness and shortness of breath, which are due to anemia, or a decreased number of red blood cells. Patients also experience bleeding or easy bruising due to thrombocytopenia, or low platelets, notes the Leukemia & Lymphoma Society. Fevers, chills and repeated infections are also common, as are swollen lymph glands and bone pain, as Mayo Clinic notes.