Haemolymph provides various kinds of protection and defence from: (i) physical injury; (ii) the entry of disease organisms, parasites or other foreign substances; and sometimes (iii) the actions of predators.

In some insects, the haemolymph contains malodorous or distasteful chemicals, which are deterrent to predators. Injury to the integument elicits a wound-healing process that involves haemocytes and plasma coagulation. A haemolymph clot seals the wound and reduces further haemolymph loss and bacterial entry. If disease organisms or particles enter an insect’s body, then immune responses are invoked. These include the cellular defence mecanisms of phagocytosis, encapsulation and nodule formation mediated by the haemocytes, as well as the actions of humoral factors such as enzymes or other proteins (e.g. lysozymes, prophenoloxidase, lectins andpeptides).

The immune system of insects bears little resemblance to the complex immunoglobulin-based vertebrate system, yet insects infected sub lethally with bacteria can acquire resistance against subsequent infection. Haemocytes are involved in phagocytosing bacteria but, in addition, immunity proteins with antibacterial activity appear in the haemolymph 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 neuropeptides may participate in cell-mediated immune responses by exchanging signals between the neuroendocrine system and the immune system, as well as influencing the behaviour of cells involved in immune reactions.

The enzyme phenoloxidase (PO) plays a central role in insect immunochemistry. It is produced as an inactive zymogen (proPO), mostly in haematocytes (oenocytoids). PO produces indoles, which are polymerized to melanin (used in encapsulation of foreign material), and the enzymatic reactions involved produce quinones, diphenols, superoxide, hydrogen peroxide and reactive nitrogen intermediates. These help defend against bacteria, fungi and viruses. Activation and inhibition of phenoloxidase involves zymogens (enzyme precursors), inhibitor enzymes, and signaling molecules in a diversity of cell types. Phenoloxidase-based immunity is metabolically expensive and has fitness costs.

The insect immune response to ingestion of pathogens is located in the alimentary system, especially the mid-gut, where two responses are induced: reactive oxygen species (ROS) and antimicrobial peptides (AMP) are produced. Management of ROS is needed to protect the gut from self-harm from peroxide and hypochlorous acids. Likewise, AMPs have to be regulated to safeguard beneficial resident microbiota, while responding rapidly and effectively against pathogenic intruders.