Lymphoblast white blood cell. Coloured scanning electron micrograph (SEM) of a lymphoblast white blood cell. Lymphoblasts, like all white blood cells, are involved in the immune system's defence mechanisms. They are a form of lymphocyte that occurs when the lymphocyte is activated by an antigen. Once activated, lymphoblasts start to divide, making copies of themselves, which will then differentiate into effector cells (plasma cells), cytotoxic T cells, and helper T cells. Magnification: x3150, when printed 10 centimetres wide.