Immunology in Fish

Innate immunity 

Acquired Immunity (adaptive Immunity/specific Immunity)

This prevents entry of micro-organisms into tissues or, once they have gained entry, eliminates them prior to the occurrence of disease.

Characteristics

• Present from birth.
  
• Non-specific – acts on many organisms and does not show specificity.
  
• Does not become more efficient on subsequent exposure to same organisms.
  

   

Important factors

A.     mechanical / physical factors

1. Keratin layer of intact skin
2. Intact mucosal lining of different organs.
3. Washing action of tears, saliva and urine.
4. Respiratory cilia- expel foreign substances including microbes.
5. Mucous layer present over the inner surfaces of the body acts as a protective barriers to block the adherence of bacteria to epithelial cells.
6. Coughing and sneezing reflexes- expel trapped foreign substance on the mucous lining of the respiratory tract.

B. Chemical factors

1. Fatty acids of skin – inhibits growth of microorganisms.
2. Acid pH of sweat and sevaceous seretions- inhibitory to micro-organisms.
3. Lysozyme in tears, nasal sections and saliva- degrades peptidoglycan, an essential element present in bacterial cell wall.
4. Spermine and zinc in the semen – are bactericidal.
5. Lactoperoxidase in milk- has bactericidal action.
6. Low pH (acidic environment) of stomach and vagina- inhibits growth of many microbes.

C. Biological factors

• Normal bacterial flora present in the throat, colon and vagina – inhibits colonization by other pathogens (bacterial interference)
  
• Phagocytosis by neutrophils and macrophages.
  

Natural killer (NK)cells – are involved in extra cellular killing of target

This type of immunity occurs in response to infection called ADAPTIVE as the immune system must adapt itself to previously unseen molecules. Following recovery from certain infections with a particular micro-organism, individuals will never again develop infection with the same organism,

but can become infected with other micro-organisms, i.e. he/she is protected against one micro-organism. This form of protection is called IMMUNITY and an individual is said to be IMMUNISED against that organism.

Cardinal features of acquired immunity

Specificity: the ability to discriminate among different type of antigenic determinants.

Memory: the ability to recall previous contact with a particular tupe of antigen, such that subsequent exposure leads to a more rapid and larger production of antibody or T cell responses.

Adaptive-ness: the ability to respond to previously unseen antigens, which may never have existed before on the earth or not defined.

Self-limitations: all normal immune responses die out with time after antigen stimulation.

Discrimination of self from non-self: lymphocytes are able to recognize and respond to many foreign antigens but are normally unresponsive to the potentially antigenic substances present in that individual.

Non-specific elimination of micro-organisms

Determinants of innate immunity

There are many factors which can modify the non-specific immune mechanisms of which following determinants are important.

Genetic factors: whole immune response is under the active control of the immune response gene. In determining the disease susceptibility, the role of genetic factors are gaining importance day by day.

There is difference in immune response among different species and races. For example, black Africans are less susceptible to develop falciparum malaria and there is also racial difference in susceptibility to other diseases like tuberculosis and coccidiiodomycosis.

Age: immune system is hypo functional in extremes of age. Therefore newborns and very old peoples are more susceptible to many infections.

Humoral and metabolic conditions of the individual: certain hormones affect immune response e.g. diabetic patients are more susceptible to bacterial infections.

Nutritional status: malnutrition is associated with lower immunity and makes the individual more susceptible to infections.

Types:

Memory cells

• are only produced in active immunity.
• Protection for active immunity is permanent whereas in passive immunity it is only temporary.
• Antigens are only encountered in active immunity.

Active immunity takes several weeks to become active but passive is immediate.

Natural killer (NK)cells

Macrophages

Polymorphonuclear leukocytes(neutrophils)

Eosinophils

These are group of lymphoid cells that are functionally,morphologically, and developmentally distinct from T and B cells and are capable of producing non-specific cellular cyto-toxicity. There are large granular lymphocytes, which produce cytotoxic effect to virally infected cells and tumor cells.

The term macrophage is derived from macro large and phage to eat, i.e. ‘large eater’. Macrophages impart very important role in both innate and acquired immunity.

These are maturephagocytic cells derived from circulating monocytes,which act primarily by engulfing and digesting bacteria, cellular debris and other particulate matter.

They function as the principal scavenger cells of the body.

Neutrophils also called microphage are short lived, phagocytic cells that contain granules filled with hydrolytic enzymes involved in innate immunity.

The y arise in the bone marrow and makes up form 60%to 70% of the total leukocytes count in the peripheral blood.

Eosinophils are distinguish by coarse cytoplasmic granules contain Eosionophils peroxidase among other enzymes that can generate toxic oxygen metabolites.

They constitute about 1-3 percent of circulating leukocytes and are not professional phagocytic cells.

They can recognize there cells when come in contact with them and forms a transmembrane pore with an annular structure and induces cellular death.

If stimulated they can release cytokines.

However, the precise physiological role played by NK cells is still not very clear. Chondroitin sufate A protects the NK cell from autolysis by its own agents.

The cells secrete enzymes, reactive oxygen radicals, nitric oxide and prostaglandin, all of which serve to kill microbes and control the spread of infection. Macrophages produce cytokines that recruit other inflammatory cells specially neutrophis and are responsible for many systemic effects of inflammation.

The primary function of this cell is ingestion and digestion of particular material especially highly virulent bacteria through the process of phagycitosis.

They take part in inflammation through the edition of certain components of complement.

This process has been termed as ‘first line of defense’.

Eosinophils are consistently found in tissues where allergic conditions are operative, such as allergic rhinitis or certain parasitic infestations.

They play a unique role in killing large parasites such as helminthes by special mechanism that cannot be physically phagocytosed.

Recognized by B cells and Ab

Recognized by T cells

Composition

• Proteins, polysaccharides, nucleic acids
• Sequence (linear) determinants
• Conformational determinants

Composition

• Proteins (some lipids)
• Sequence determinants
  • Processed
  • MHC presentation (lipid presentation by MHC-like CD1)

Size: 4-8 residues 

Size: 8 -15 residues 

Number

• Limited (immunodominant epitopes)
• Located on the external surfaces of the Ag

Number

Limited to those that can bind to MHC