Fluorescence light micrograph of lamellocytes from a fruit fly (Drosophila melanogaster). Lamellocytes are a cell of the fruit fly's immune system. The presence of a large pathogen such as a parasitoid wasp egg in the fruit fly's haemocoel activates lamellocytes and they encapsulate the pathogen. Plasmatocytes measure between 20 to 30 micrometres across.