Re: what makes a cell with leukemia function differently from a healthy cell?

In a normal, healthy cells, DNA (deoxyribonucleic acid) dictates its role and functions such as what kind of cells it will be, when it will mature, and how long it will live. When DNA is lost or damaged (which may be due to factors such as radiation, chemicals, viruses infection, carcinogens etc.), the cell goes hay-wired. In the case of leukemia, one or a few white blood cells with lost or damaged DNA begin to multiply (proliferate) without control; they neither mature nor die. These abnormal cells accumulate in the body and block production of red blood cells (essential for delivering oxygen), platelets (essential for clotting), and normal white blood cells (essential for body's immune system). Normal white blood cells combat infections, but leukemic white cells are generally poor infection fighters.

see also:

What's leukemia
Treatments
Leukemia Society of America