Fish Immunology (1+1)

Unit 13 - Defence mechanisms in crustaceans

13.1. Defence mechanisms in crustaceans
Aquatic animals are surrounded by microorganisms in aquatic environment that are capable of initiating infection in these animals. Unlike fish that possess both innate and adaptive immunity, the lower aquatic animals such as shrimps that are invertebrates lack true adaptive immune system and they depend upon their innate immune system for the defense mechanism against the invading pathogen. In normal conditions, crustaceans maintain a healthy state by mounting a defense reaction against potential pathogens. Among them, the external cuticle is a first line of defense to provide an effective physical and chemical barrier against the attachment and penetration of pathogens. The digestive tract, which is a main route of invasion of the pathogen, is partly lined with chitin layers and its hostile environment of acids and enzymes is able to inactivate and digest many bacteria and viruses.
Crustacean hemocytes (blood cells) play a central role in their immune reactions and are capable of phagocytosis, encapsulation, nodule formation and mediation of cytotoxicity. When the host defence encounters a pathogen, a series of pathways get activated to protect crustacean against infection by a variety of microorganism. The entry of a pathogen into the hemocoel of the host triggers a complex system of innate defense mechanisms involving cellular and humoral immune components.
Cellular immune components are :-

• Hemocytes
• Phagocytosis
• Encapsulation
• Clotting reaction
• Prophenoloxidase (proPO) system

Humoral immune components include:-

• Lysozymes,
• Anti-microbial peptides and
• Lectins