Tumors of the hematopoietic and lymphoid tissues
A doctor will do some tests to find out if a person has a blood cancer.  One common test is a complete blood count.  The cells that circulate in the bloodstream are generally divided into three types: white blood cells (leukocytes), red blood cells (erythrocytes), and platelets (thrombocytes).  The relative proportions is what picks up indications of a problem.