Fish Immunology (1+1)

Unit 13 - Defence mechanisms in crustaceans

13.2. Cellular immune components
Hemocytes
Hemocytes present in the circulating hemolymph play a crucial role in invertebrate immunity providing protection against invading microorganisms. Three types of circulating hemocytes can be found in the crustaceans, they are hyaline (H) cells, semigranular (SG) cells and granular (G) cells.
Hyaline (H) cells
Hyaline cells are small, spherical and are least in numbers among all hemocytes. The number of hyaline cell population varies between different crustacean species. Hyaline cells are capable of phagocytosis and destruction of invading pathogens.
Semigranular cells
These semigranular cells have variable number of small eosinophilic granules that are mainly responsible for encapsulation and also contain the prophenoloxidase activating systems (proPO).
When pathogens or any foreign particles invade into the body these semigranular cells activate and degranulate. After degranulation these cells attach and spread on the foreign particles.
Granular cells
These cells are filled with large number of granules in their cytoplasm. These cell do not participate in phagocytosis but involved in cytotoxicity activity, nodule formation, proPO system and sometimes involved in encapsulation process.
Phagocytosis
The process trapping, ingestion, destruction and elimination of pathogen or foreign body from host tissue by the phagocytic cells are called "Phagocytosis". The mechanism of intracellular destruction of microorganisms through phagocytosis in shellfish is similar to other animals.
Phagocytes are of two types

• Fixed phagocytes
• Mobile or circulating phagocytes

Fixed phagocytes
These fixed phagocytes are fixed in certain places in the tissue of the organs. These fixed phagocytes can be observed in the lacunae of gills, pericardial sinuses and base of appendages.
Mobile or circulating phagocytes
These phagocytes can be seen in haemolymph with continuous circulation. These phagocytes localize in the site of infection and destroy the pathogen by the process of phagocytosis.