An ultrastructural and autoradiographic study of the immune response in Hyalophora cecropia pupae.

Three types of hemocytes are found in the Cecropia pupa, plasmatocytes, granular cells, and spherule cells. The granular cells are the major phagocytic blood cells, taking up the bacterium Enterobacter cloacae when this is injected into the pupae. Disintegrating blood cells are observed near the pericardial tissue. No other changes in ultrastructure are noted in hemocytes and pericardial cells which could be correlated with the immune response in these pupae. The fat body cells from pupae injected with bacteria contain abundant RER and Golgi bodies, whereas those from wounded (saline injected) and untreated controls do not. The fat body is the only tissue that responds to bacterial injection by increased incorporation of 3H-uridine into RNA. These findings support the idea that the fat body is the main site of synthesis of the immune proteins.