Humoral immunity

HUMORAL IMMUNITY

Humoral immunity is mediated by factors present is the haemolymph (Blood of crustaceans). Some of the important humoral factors involved in crustacean immunity are

Pattern recognition proteins (PRPs )

  • Crustaceans do not have immunoglobulins and rely on a non- adaptive immune defence for this purpose. This defense is initiated by recognition of a broad spectrum of factors that are released or present on the surface of microorganism or parasites. The protein that recognize these factors are called pattern recognition proteins (PRP). PRPs are involved in the recognition process of cell wall components from microorganism, such as lipopolysaccharides (LPS) and β-1-3-glucans (BG) are called LPS-binding protein and β glucan–binding protein (BGBP), respectively. The functions of these pattern recognition proteins is to selectively bind to factors present on bacterial and fungal pathogens and then to haemocytes. However, these PRPs are unable to destroy foreign matter, and a cellular activity, mainly phagocytosis is required.

Lectins

  • Lectins are proteins or glycoproteins usually without catalytic activity that has the ability to bind to specific carbohydrates expressed on different cells. They occur on the surface of viruses, bacteria, yeast, and protozoan and throughout all animal and plant kingdoms. The type of carbohydrate to which they bind always determines their specificity. Lectins are also involved in cellular and tissue transport of carbohydrates, glycoproteins and calcium, cell adhesion, migration and apoptosis. Multiple plasma lectins, each with different carbohydrate-binding specificities and consequently, able to potentially recognize different microorganisms, such as yeast (Sacharomyces cerevisiae) and bacteria (Escherichia coli and Bacillus cereus) has been reported. This response is based on the sugar specificity of each lectin. The lectin molecules also have ligands for receptors on the surface of hemocytes to facilitate opsonisation. The lectins also activate the proPO system.

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Antibacterial peptides

  • The production of antimicrobial peptides is a first-line defense mechanism in innate immunity. Three antimicrobial peptides from the haemolymph and haemocytes of shrimp have been characterized. Penaeidins are the antibacterial peptides present in penaeid shrimp. The penaeidins had no effect on gram-negative bacteria such as Vibrio . The antibacterial activity is predominantly directed against gram-positive bacteria with different specificities in their mode of action.

The proPO system

  • Microbes and parasites can naturally enter into the crustaceans through wounds or by contamination of the food. The response to this can often be seen as dark spots in the cuticle of the crustaceans and intruders become brown black. This colour is caused by melanin which is produced as a product of the prophenol oxidase system (proPO) system of arthropods. The enzyme responsible for the melanin formation is phenoloxidase (PO), which catalyzes the oxidation of phenols to quinines that subsequently polymerize into melanin. During the formation of melanin, toxic metabolites are formed which have fungistatic activities.
Last modified: Sunday, 18 September 2011, 5:24 AM