A null cell is a large agranular lymphocyte that is developed inside of bone marrow. Null cells lack the common characteristic surface markers that can be found in mature B-cells and T-cells. They are stimulated when an antibody is present and attack viral-infected or tumor cells. Null cells make up a small proportion of the lymphocytes that can be found in an organism. Because the term null cell is no longer commonly used, they are often referred to as natural killer cells or killer cells.