Lacking an adaptive immune system, shrimps rely on their effective cellular and humoral innate immune responses to combat invading microbes.
The cellular immune reactions include phagocytosis, nodulation and encapsulation, whereas the humoral responses involve the synthesis and release of several immune proteins, such as antimicrobial peptides (AMPs), proteinase inhibitors, cytokine-like factors, etc.
In crustaceans, including shrimps,major immune reactions take place in hemolymph, which contains three different principal types of hemocytes that are defined as the hyaline, granular and semigranular hemocytes.
Several immune molecules are produced and stored in the granules of hemocytes before being released into the hemolymph upon activation by bacterial and/or fungal cellwall components, such as peptidoglycan (PG), lipopolysaccharides (LPS) and b-glucans (BGs).
Pattern recognition proteins (PRPs) or pattern recognition receptors (PRRs) recognize and bind the microbial cell wall components and activate various immune responses.