Fish Immunology (1+1)

Unit 13 - Defence mechanisms in crustaceans

13.4. Humoral defence mechanism
The humoral defence mechanism is a defence mechanism in which the direct cellular involvement is absent. Instead the cellular products like Lysozymes, Anti-microbial peptides and Lectins can be seen involved.
Lectins
Lectins protect the animals against infection. Lectin, or agglutinin, is a carbohydrate binding protein that can bind to the carbohydrate on the cell wall of pathogens or foreign bodies, causing the reaction known as agglutination. Lectins are produced in hemocytes and some of them are transported on the membrane surface and here they participate in recognition of foreign molecules or participate in carbohydrate transportation. Lectins can induce agglutination that can lead to phagocytosis and lectins can also act as opsonins. Lectins are present in almost all crustaceans.
Antimicrobial peptides (AMPs)
The antimicrobial compound is a part of humoral defense system in crustaceans that plays an important role in innate immunity. Antimicrobial peptides (AMPs) which are 150-200 amino acids long are produced by different types of cells. A family of AMPs called penaeidins displaying both antifungal and antibacterial activities is present in hemolymph of the shrimp. Anti-lipopolysaccharide factors (ALFs) have been recently identified from hemocytes of P. monodon, and this ALF has a broad spectrum of antifungal properties and antibacterial activities against both Gram-positive and Gram-negative bacteria. Another antimicrobial peptide named crustin a peptide present in Carcinus maenas of 11.5 kDa size has an antibacterial activity against Gram-positive bacteria.
Lysozymes
The haemocytes and haemolymph contain lysozymes that can act against the pathogen. Once the antigen or pathogen enters the body cavity, it will stimulate the haemocytes to synthesis the higher quantities of lysozyme followed by release of these enzymes into the haemolymph where these enzyme will act on pathogen or susceptible material and destroy them.