Antibody and cell mediated immunity of fish and shellfish

1. ANTIBODY AND CELL- MEDIATED
IMMUNITY OF FISH
AND SHELLFISH
DEFENCE MECHANISMS IN FISH
Submitted by
B.Naveen Rajeshwar
Dept of AAH

2. INTRODUCTION
• Host defense mechanisms consist of innate immunity and
adaptive immunity.
Nonspecific immunity
Specific immunity
• Innate immunity also called natural or native or nonspecific
immunity, takes part in the initial protection against Pathogens.
• Adaptive immunity also called specific or acquired immunity
take part after innate immunity, adaptive immunity is even more
effective against pathogens.
• Adaptive immunity is stimulated by the presence of pathogens.

3. SPECIFIC DEFENSE MECHANISM IN FISH
• Specific immune system can be seen in vertebrates which is also
referred as advance defense system in organisms.
• Specific immune system is made up of two cellular systems:
Humoral antibody system (B cells)
Cell-mediated immunity (T cells)
• In specific immunity, the important aspect is memory factor.

5. HUMORALANTIBODY SYSTEM (B CELLS)
• Humoral immune response acts actively against free microbes those
circulate in blood and present in extracellular spaces, but does not
provide adequate protection against intracellular pathogens
including bacteria, virus, protozoa and fungus.
• Humoral antibody uses B cells where the B cells differentiate into
antibody producing Plasma cells and Memory cells.
• Series that involved in stimulating B cells to produce antibody are:-

7. 1)ANTIGEN PRESENTATION
• The antigen-presenting macrophages recognize the pathogen engulf
the pathogen, digests it and displays antigen on their surface.
• The processing involves proteolysis, which presumably occurs within
acidic subcellular compartments.
• These macrophages display residual foreign antigen on the cell
membrane and this is l0 times more effective than unbound antigen
in promoting immune response.
• The macrophages also secrete a protein called interleukin- 1 (IL- 1)
which activates the helper-T cells.

8. 2)HELPER T CELLS
• These T cells are activated when macrophages-bound antigen
linked to class II Major Histocompatibility Complex (MHC) in the
presence of interleukin-l and they secrete two factors like B cell
growth factor (IL-4) and B cell differentiating factor (IL-5).
• The IL- l, IL-4 and IL-5 act together to provoke B cell to respond
quickly.

9. 3)B CELLACTIVATION
• The membrane receptors for antigen are concentrated at a particular position on
the surface of the B cells.
• The B cells divide repeatedly to form antibody-secreting plasma cells and
memory cells.
• These plasma cells are widely distributed throughout the body but concentrated
in the head kidney and spleen of fish.
• The plasma cells usually die after 3-6 days of secretion
• The second populations of cells derived from antigen- sensitive B cells and
morphologically indistinguishable from parent cells are known as memory cells.
• The interactions between an antigen-presenting cell, a B cell and a T-helper cell
that lead to the activation of B cell and development of memory cells and
antibody-producing plasma cells.

10. CELL-MEDIATED IMMUNITY (T CELL)
• Cell mediated immunity provide adequate protection against
intracellular pathogens by the lymphocyte T cells which originate
from stem cells of kidney and mature in thymus.

11. T CELLANTIGEN RECEPTOR:
• Unlike B cells, the T cell lack the surface immunoglobulin and
respond to a foreign antigen when it closely associated with a class I
or Class II MHC antigen or another cell.
• Both T cell and target cells must have identical histocompatibility
antigens.
• In most of the fish, the T cell receptor is a digomeric complex with
two functional subunits i.e., ligand binding and signal transducing.
• Each consists of two polypeptide chains such as alpha and beta.
• Each chain of alpha and beta units composed of four domains. Two
extracellular, a hydrophobic transmembrane domain and a C-terminal
domains.

12. FUNCTIONAL CLASSIFICATION OF T CELLS
• Functionally, the T cells are classified into four subsets such as helper (Th) cells,
Cytotoxic (Tc) cells and T cells with suppressor (Ts) and delayed hypersensitivity
(Td) activities.
 Helper T (Th) cells recogbize the antigen in association with class II MHC on
surface of the antigen presenting cells.
 Cytotoxic T cell (Tc) recognise the antigen in association with class I MHC.
 Delayed hypersensitivity T cell (Td) recognizes antigen in association with
either class I or class II MHC.
 T suppressor cells (Ts) recognize antigen assocaited with class II MHC and they
secrete factors that regulate the immune response.

14. EFFECTOR FUNCTION OF T CELLS
• During the recognition process the antigen presenting macrophage
secrete IL-1 and the IL-2 is retained by helper T cells.
• The effector T cell is activated generating both the memory and
effector cells population.
• These effector cells called lymphoblasts, secrete numerous
glycoproteins known as lymphokines, which enhance the
bactericidal activity of phagocytes.
• These cells also particiapate in direct cytotoxic reactions and able to
kill infected host cells and thus reduce the ability of intracellular
pathogens to prolifereate within T lymphocytes.

15. DEFENCE MECHANISMS IN SHELLFISH
• Unlike fish, the lower aquatic animals such as shrimps that are
invertebrates lack true adaptive immune system.
• They depend upon their innate immune system for the defense
mechanism against the invading pathogen.
• The external cuticle is a first line of defense.
• The digestive tract is a hostile environment of acids and enzymes is
able to inactivate and digest many bacteria and viruses.
• The entry of a pathogen into the haemocoel of the host triggers a
complex system of innate defense mechanisms involving cellular and
humoral immune components.

16. Cellular immune components are:-
• Haemocytosis and haemocytic infiltration
• Phagocytosis
• Encapsulation
• Clotting reaction / thrombosis
• Nacrezation / Prophenoloxidase (proPO) system
Humoral immune components include:-
• Lysozymes,
• Anti-microbial peptides and
• Lectins

17. CELLULAR IMMUNE COMPONENTS
Haemocytes
Haemocytes present in the circulating hemolymph play a
crucial role in invertebrate immunity providing protection against
invading microorganisms. Three types of circulating haemocytes can
be found in the crustaceans, they are
Hyaline (H) cells,
Semigranular (SG) cells and
 Granular (G) cells.

18. • Hyaline (H) cells
Hyaline cells are small, spherical and are least in numbers among all
haemocytes. Hyaline cells are capable of phagocytosis and destruction of invading
pathogens.
• Semigranular cells
These semigranular cells have variable number of small eosinophilic
granules that are mainly responsible for encapsulation and also participate in the
the prophenoloxidase activating systems (proPO).
• Granular cells
These cells are filled with large number of granules in their cytoplasm.
These cell do not participate in phagocytosis but involved in cytotoxicity activity,
nodule formation, proPO system and sometimes involved in encapsulation
process.

19. Phagocytosis
The process trapping, ingestion, destruction and elimination of pathogen
or foreign body from host tissue by the phagocytic cells are called "Phagocytosis".
The mechanism of intracellular destruction of microorganisms through
phagocytosis in shellfish is similar to other animals. Phagocytes are of two types;
 Fixed phagocytes
These fixed phagocytes are fixed in certain places in the tissue of the
organs. These fixed phagocytes can be observed in the lacunae of gills, pericardial
sinuses and base of appendages.
 Mobile or circulating phagocytes
These phagocytes can be seen in haemolymph with continuous
circulation. These phagocytes localize in the site of infection and destroy the
pathogen by the process of phagocytosis.

20. Encapsulation
When the pathogen like parasites which are too big to engulf by
phagocytes invade the crustaceans, the haemocytes present in haemolymph
collaborate and deposit on the surface of the parasite and block off the parasite
from circulation. Normally the cells involved in the encapsulation is semigranular
cells.
Cytotoxicity
Haemocytes are capable of destroying tumor and non-tumor cells
present in the host by secreting cytotoxic molecules. Cells that are mainly
responsible for cytotoxicity is Granular and semigranular cells which are similar
to mammalian natural killer cells (NK cells). Once these cytotoxic cells recognize
the foreign particle or tissue, they attach to the foreign particle or tissue and
release the cytotoxic chemicals, that kills the target.

21. Clotting reaction
Clotting reaction is the mechanism of formation of extracellular
and intravascular clots for rapid sealing of wounds. Clotting prevents the
loss of haemolymph and invading pathogens. Intravascular clots produce
stasis by adhering to the walls of blood vessels and extracellular clots
inhibit movement of pathogens and make them vulnerable to phagocytosis.
• Two different coagulation mechanisms
Haemocyte derived clotting cascade – Horse shoe crab
Transglutaminase dependant clotting reaction- Shrimp, Lobster, Crab

22. Prophenoloxidase (proPO) system
• This system is involved in proPO mediated melanization process using prophenoloxidase
(proPO) enzyme.
• proPO is synthesized in the hemocytes and are localized in the granules of the hemocytes.
• When antigen binds with receptor, it becomes activated and binds specifically to a cell
surface associated protei.
• The recognition of non-self materials triggers degranulation of semigranular and granular
cells.
• Prophenoloxidase activating enzymes (proppA) is released during the process which is
further activated to ppA by the pathogen.
• Active ppA converts the proPO to phenoloxidase. PO is a bi-functional copper containing
enzyme, which is known as a tyrosinase and catalyzes the early steps in the pathway to
melanin formation.
• Melanin is a pigment which is black in color and helps the host to neutralize the pathogen.

23. 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.
• It binds 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.
• Lectins are present in almost all crustaceans.

24. Antimicrobial peptides (AMPs)
Antimicrobial peptides (AMPs) which are 150-200 amino acids long are
produced by different types of cells.
Penaeidins displaying- antifungal and antibacterial activities (Shrimp
hemolymph)
Anti-lipopolysaccharide factors (ALFs) (hemocytes of P. monodon).
Crustin- antibacterial activity (Carcinus maenas) of 11.5 kDa size
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.

25. CONCLUSION
• Nonspecific Immunity
 Humoral factors - AMP’s, Cytokines, Interferons, etc
 Cellular factors – Macrophage, Neutrophil (Phagocytes) Haemocyes
(invertebrates)
• Specific Immunity
 Humoral System (B cells)
 Cell mediated immunity (T cells)

26. Thank You…!