Leukemia, or blood cancer, is characterized by an abnormal increase in white blood cells. White blood cells are responsible for fighting infections and diseases in the body. In leukemia, there is a disruption in the normal production and function of these cells, leading to their overproduction. This excessive production of white blood cells can crowd out other healthy cells in the bone marrow, leading to a decrease in the production of red blood cells and platelets. However, the primary abnormality in leukemia is the increase in white blood cells. Therefore, option 2, white blood cells, is the correct answer. It is important to note that leukemia can present with different subtypes, each having different characteristics and affecting different types of white blood cells.