Fish Diseases and Management

3.2.2.3.Mechanisms of cellular defence in crustaceans

1. Phagocytosis

Phagocytosis is the process of internalisation of invading pathogens and other small foreign particles by individual blood cells. This is the first line of defence in crustaceans against the pathogens, which invades the tissue bypassing the cuticle. The phagocytosis is carried out mainly by the hyaline cells. Phagocytic activity is enhanced by the humoral factors present in the haemolymph which sensitises the foreign particles / cells apart from agglutinating and immobilising them.

2. Nodule formation

Following to the heavy load on phagocytosis, the nodule formation is activated. In nodule formation, the microorganisms are trapped by several layers of haemocytes and subsequent melanization of the nodule. The haemocytes form clumps around the invading pathogens.  These clumps consist of several layers of flattened cells many of which undergo degranulation leading to melanization. The pathogens are removed from the circulating haemolymph of the host by localisation due to nodule formation in gills and sinuses in the hepatopancreas.

3. Encapsulation

This process is carried out by the host when the parasite is too large to be engulfed by phagocytosis. Semi-granular cells are primarily involved in the process. Encapsulation is mainly directed against  fungal hyphae or helminth or nematode parasites. Following encapsulation by a process similar to nodule formation, melanin is deposited between the cells and the foreign surface. Due to the antimicrobial property of  melanin and the precursor quinones, the pathogens are immobilised and contained.

4. Cytotoxicity

The invertebrate blood cells can affect cytotoxic reactions resulting in the destruction of the foreign cells, but the mechanism and significance remain unknown. The cytotoxic  reactions are believed to be due to the direct contact of haemocytes with the invading cells.

5. Cell adhesion

Cell adhesion is essential in invertebrate immunity in the cellular immune responses of encapsulation and nodule formation. Blood cell adhesion to the infectious organism occurs when they move to and attach at a site of inflammation or infection. In the immune response of invertebrates, blood cells adhere to large parasites in the process of encapsulation or to each other to from nodules in the presence of microorganisms.

Haemocytes of crustaceans on degranulation release a cell-adhesive and opsonic peroxidase, peroxinectin that supports cell adhesion, stimulates phagocytosis, stimulates encapsulation and induces peroxidase activity. A site containing the motif, KGD (Lys-Glys-Asp), appears to be adhesive by binding to a transmembrane receptor of the integrin (integral plasma membrane protein) family on the blood cells. Peroxinectin also binds a peripheral blood cell surface CuZn-superoxide dismutase.