When pathogens invade the insect hemocoel (body cavity) they immediately confront two major forces: immune-responses and circulatory currents. The immune response is mediated by circulating and sessile hemocytes, the fat body, the midgut, and the salivary glands. These tissues drive cellular and humoral immune processes that kill pathogens via phagocytosis, melanization, lysis, encapsulation, and nodulation. Moreover, immune-responses take place within a three-dimensional and dynamic space that is governed by the forces of the circulatory system. The circulation of hemolymph (insect blood) is primarily controlled by the wave-like contraction of a dorsal vessel, which is a muscular tube that extends the length of the insect and is divided into a thoracic aorta and an abdominal heart. Distributed along the heart are valves, called ostia, that allow hemolymph to enter the vessel. Once inside the heart, hemolymph is sequentially propelled to the anterior and to the posterior of the body. During an infection, circulatory currents sweep small pathogens to all regions of the body. As they circulate, pathogens encounter immune factors of the insect that range from soluble cytotoxic peptides to phagocytic hemocytes. A prominent location for these encounters is the surface of the heart. Specifically, periostial hemocytes aggregate in the extracardiac regions that flank the heart's ostia (the periostial regions) and phagocytoze pathogens in areas of high flow of hemolymph. This review summarizes the biology of the immune and circulatory systems of insects, including how these two systems have co-adapted to fight infection. This review also compares the immune and circulatory systems of insects to that of crustaceans, and details how attachment of hemocytes to cardiac tissues and the biology of the lymphoid organ demonstrate that dynamic interactions between the immune and circulatory systems also occur in lineages of crustaceans.
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