VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

CELL MEDIATED IMMUNITY

The clotting system

• The crustaceans cuticle is an effective barrier against microbes, but all animals will get small or large wounds during their lifetime. This could be an opportunity for microorganisms to easily enter into the animal if it did not have a rapid and efficient clotting system. Upon wounding of the cuticle, there will be a rapid aggregation of haemocytes at the site of the wound and a clotting reaction will take place in order to seal the wound.

Haemocytes

• Even with a very quick clotting system that seals of the wounding site from the blood fluid and tissues, some microorganisms can penetrate into the circulation and must be neutralized. As in vertebrates, the invertebrate blood cells are very important in dealing with these invaders. They are responsible for phagocytosis and clearance of particles that have entered the blood system as well as for encapsulation of larger bodies such as helminthes or protozoans. In crustaceans there are three major cell type in the circulatory system; the number of each cell type may, however, vary in different species.
  • Semigranular cells: These are the more sensitive cells, and the ones that first react against foreign organisms. They have only a few granules, but these granules contain the proPO-system which will be released and activated. The semigranular cells can also conduct some limited phagocytosis.
  • Granular cells: These cells have large granules and are responsible for storing the major amount of the proPO-system components.
  • The semigranular and granular cells are also involved in encapsulation of foreign particle, although the semigranular cells can seem to be more active in this process than the granular cells.
  • Hyaline cells: This is the major phagocytic cell. By engulfing microorganisms, the hyaline cell can use more destructive processes for killing the microbe and have it restricted to the compartment of the cell. Such processes could include release of lysosomal enzymes or generation of reactive oxygen species and nitric oxide (NO).