Protection and defense by the hemolymph

Hemolymph provides various kinds of protection and defense from (a) physical injury; (b) the entry of disease organisms, parasites, or other foreign substances; and sometimes (c) the actions of predators. In some insects the hemolymph contains malodorous or distasteful chemicals, which are deterrent to predators (Chapter 14). Injury to the integument elicits a wound-healing process that involves hemocytes and plasma coagulation. A hemolymph clot is formed to seal the wound and reduce further hemolymph loss and bacterial entry. If disease organisms or particles enter an insect's body, then immune responses are invoked. These include the cellular defense mechanisms of phagocytosis, encapsulation, and nodule formation mediated by the hemocytes, as well as the actions of humoral factors such as enzymes or other proteins (e.g. lysozymes, prophenoloxidase, lectins, and peptides).

The immune system of insects bears little resemblance to the complex immunoglobulin-based vertebrate system. However, insects sublethally infected with bacteria can rapidly develop greatly increased resistance to subsequent infection. Hemocytes are involved in phagocytosing bacteria but, in addition, immunity proteins with antibacterial activity appear in the hemolymph after a primary infection. For example, lytic peptides called cecropins, which disrupt the cell membranes of bacteria and other pathogens, have been isolated from certain moths. Furthermore, some neuro-peptides may participate in cell-mediated immune responses by exchanging signals between the neuroendocrine system and the immune system, as well as influencing the behavior of cells involved in immune reactions. The insect immune system is much more complicated than once thought.

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