DEFENSE MECHANISM IN FINFISH AND SHELLFISH

Introoduction

Farming of fish and shellfish has gained significant grounds in several parts of the world .

Now a days disease is main problem in aquafarming.

In recent years ,lot of attention is being given to health management using various forms of immunoprophylactic techniques such as vaccination and immunostimulation .

To reduce immunoprophylactic application ,it is vital to have insight into specific and non specific defense mechanism of farmed animal .

Through disease process studies ,it is very well known that a pathogen can cause disease only if it can overcome the non specific and specific defense barriers of the host and successfully establish and proliferate

The immune system of fish is very similar to vertebrates, although there are some important differences.

The immune system ,vested with the role of defense is composed of various cell type ,tissue and organ.

The use of these products reduces the need for therapeutic treatments, enhances the effects of vaccines and, in turn, improves the indicators of production.

Types of defense system

1.Non specific

2.Specific immune system

The former is encounter in almost all living organism including fishes .It is non specific because the same immune response can be elicited by number of unrelated foreign particles .

Specific or acquired mechanism of immunity is found only in vertebrate and the reaction are directed against specific molecules that stimulate such reaction

Though these two mechanism appear as distinct in fact they function in conjunction with each other ,making the study of immunology a complex and vast one.

Non –specific defense mechanism

Non adaptive immunity/ innate immunity The main determinants of innate immunity are

genetically controlled varying widely with species ,strain and to a lesser extent between individuals.

Some of the important components of non specific defense system

1. integumental 2. phagocytes 3.tissue and serum 4. other cells 5.Inflammatory response 6.Encapsulation

1 . Integumental : skin and mucus act as primary surface barriers to several pathogenic agents. Continuous production and sloughing of mucus keep the fish skin and gills free from pathogen colonization

In addition , lytic property of mucus due to lysosomal enzyme prevent colonization of skin and gills by the pathogen .Low ph and mucus of alimentary tract offer similar protection against colonization of gut by enteric pathogen

2. Fish have fixed and circulating phagocytes ,which act as the main line of defense against pathogen which breaches the primary line of integumentary barriers . Reticulo endothelial system (RES)present in kidney ,spleen ,and heart ,phagocytes destroy pathogen and also remove dead host tissue .

Several serum factors antibody and complement act as opsonins and enhance the rate of phagocytosis .

Chemo tactic factors like macrophage aggregation factor(MAF)and macrophage migration inhibition factor (MIF) enhance the action of phagocytes at the site of injury..

3. Tissue and serum components A) c-reactive proteins (CRP) found in the serum

bind to phosSphoryl ester group and exhibit their antibacterial and antifungal activity.CRP also act as opsonins and enhance phagocytosis

B) Complement present in the serum are activated by the classical and alternate pathway. In the classical pathway antibody antigen complex activates the complement system are involved in lysis ,chemo taxis

C) Proper din- a protein present in fish serum is activated by cell wall product of bacteria and is involved in the activation of complement through the alternate pathway .

D) Lysosomal enzyme present in mucus ,serum ,and phagocytes cause lysis of pathogen .

E) Natural agglutinin and precipitin present in serum and mucus agglutinate and precipitate particulate and soluble antigen respectively.

F) Transferrin starves bacteria by binding iron in the serum.

G) Interferon produced by viral infected cells protects the other potential host cell.

H) Lymphokines produced by t –lyphocytes specifically in response to antigen have non specific effect on inflammation and phagocytosis at the site of injury.

4. Other cells; A)mast cell ; fish may not possess true

mast cell similar to the ones present in mammal, Histamin contaning cells, such as eosinophilic granular cells(EGC) present in integument ,gills and gut may probably be mast cell analogue in fish

  B)natura l killer cell /non specific

cytotoxic cell(nk/nc cell)are belived to have a role in immobilizing and killing viral infacted cells and pathogen like i.Mullifilis

 

5. Inflmmatory response : it is a well developed, non –specific, vascular response aimed to destroy ,dilute or wall off the cause of injury .

Inflammation is initiated as long as the functional blood supply is present at the injured area .Inflammation is characterized by reddining swelling pain, heat and loss of function.Vasoactive amines released following tissue damage exert a significanes influence on the microcircultion of the area .

6 encapsulation : in this type of response fibrous tissue composed of collagen encapsulation the large parasite pathogen such as digenetic trematodes and cestodes . 

specific defense system (adaptive immunity)

Specific defense system in all vertebrate including fish is characterized by specificity and memory.

Lymphocytes are central to specific defense system . There are two main population of lymphocytes namely

B-lymphocytes and T –lymphocytes. Fish have three major lymphoid organ

thymus ,anterior kidney and spleen. Thymus is regarded as the primary lymphoid organ. Thymus is the first organ to become lymphoid and

provided the virigin pool of lymphocyte to the secondary lymphoid organ namely kidney and spleen .

The lymphoid population in the secondary lymphoid organ perform the vital function of immune responsiveness such as antigen recognition .

The specific defense system has two arms 1 . Humoral immunity(B-cell) 2. Cell mediated immunity system(T-cell)

 

1. Humoral immunity B-lymphocyte differentiate into plasma cells

which produce antibody specific to the stimulating antigen and into memory cells which are capable of becoming plasma cells on subsequent exposure to the same antigen .

Antibody neutrolise pathogen by agglutination ,percipitation neutrolisation and complement activation .

2. Cell mediated immunity(CM Different population of t- lymphocyte namely T- helper

cells T-supressor cells , T-killer cells and T-lymphokinin producing cells are involved in the execution of CMI .This aspect of the immune response is wide ranging and also recruit the macrophages which constitute the body’s main line of non specific defense

Different function of T-lymphocyteT-helper cellsT-killer cellsT-lymphokine producing cellsT-supressor cells

Defense mechanism in shell fish

introduction

Crustacean encounter a variety of pathogen in the environment they live in

Though the specificity of crustacean defense mechanism is not comparable to that of higher vertebrate or even fish ,the efficiency with their defense system overcome the pathogenic hostility is highly remarkable .

In crustacean nonspecific immune system has a greater role to play due to a poorly developed specific immune system with a very weak or no memory function in it.

The defense in crustacean is largely dependent on the blood cells and haemolymph activities

Lot of interest is being generated towards understanding the basis of defense mechanism in shrimp so as to develop practical immunoprophylactic management tool to contain disease .they have well developed specific immune system with no specific and memory

One recent study has been describe that shrimp do have memory i.e they showed enhance response to booster immunostimulation by glucan and there is possibility of maternal transfer of immunity .shrimp do recognize self/non self

defense system of crustacean

A) MOBILE DEFENSE

B) FIXED DEFENSE

C) PRO-PHENOLOXIDASE SYSTEM (PPO)

a) fixed defense Hard cuticle :- hard exoskeleton of crustacean acts as first

line of defense being structural and chemical barriers to several pathogen .therefore ,the most vulnerable phase of the disease in the life cycle of crustacean is the molting stage.

During this stage parasite find it easy for attachment and penetration which also leads to other infection and complexities

Bronchial podocytes act as fixed phogocytic cells and remove pathogen from hemolymph. Crustacean have remarkable ability for autotomy and regeneration of appendages .

Clotting and wound repair is rapid and well developed in crustacean.

B) MOBILE DEFENSE 1. Cellular2. Humoral

1. cellular : crustacean have an open circulating system having analogues of white blood cells ,called hemocytes.crustacen have three types of hemocyte namely hyaline cells ,semigronulocytes and granulocytes.hyoline cells are primarily responsible for phagocytosis while semi granulocytes and granulocytes perform the function of encapsulation and nodule formation

a) phagocytosis b) encapsulation c) nodule formation d) cytotoxicity

2. Humoral factor Humoral factor include large no. of lectine,lysine,agglutinin. Lectine bind to carbohydrate of foreign cells. Agglutini can help in adhesion of heamocyte to foreign cells

C) prophenol oxidase system(PPO)