Lymphocytes and flagelate protozoan. Coloured scanning electron micrograph (SEM). Lymphocytes are a type of white blood cell involved in the immune system. Protozoa are single-celled (unicellular) micro-organisms that are eukaryotes, meaning they have a cell nucleus. They generally have animal-like behaviour, such as being mobile. Magnification: x2000 when printed at 10 centimetres wide.