This article is a more specific discussion of ALL. Please be sure to read Leukemia: The Basics first, so you have a basic understanding of leukemia.What is ALL? ALL is a blood cancer that affects the white blood cells. There are two main types of white blood cells, lymphocytes (affected by ALL) and myelocytes (affected by AML). Lymphocytes are further broken down into B and T cell lymphocytes. ALL is a spectrum of disease comprised of several different subtypes, named for the cell type that is affected (B or T) and how abnormal the cell appears under a microscope. A person with ALL develops abnormal numbers of white blood cells rather quickly, usually over weeks, giving the disease the name "acute". The white blood cell (WBC) count may be higher or lower than normal, but the WBCs that are being produced are immature and do not function well. Because WBCs are an ... read more on Acute Lymphoid Leukemia (ALL)