Immunity

Immunity is the resistance exhibited by the host against any foreign antigen including microorganisms.

2 Types of immunity

• Innate
• Acquired

1.Innate Immunity

It is the resistance which the individual possesses by birth. It is not dependent on prior contact with antigens.

Types of innate immunity

• Species immunity: Resistance to a pathogen shown by all members of a particular species .Eg B.anthracis infects humans but not chickens.
• Racial immunity; Different races exhibit resistance or susceptibility to infections.
• Individual immunity: Resistance to infection varies with different individuals of the same race and species .

Factors influencing Innate immunity

• Age: Children and elderly have a higher susceptibility to infections.
• Hormones: Hormonal disorders like diabetes mellitus, hypothyroidism enhance susceptibility to infections.
• Nutrition :Malnutrition predisposes to infections.

Mechanism of Innate immunity

1. Epithelial surfaces:

• Skin: It acts as a mechanical barrier. It also provides bactericidal secretions which are present in the sweat and sebaceous glands.
• Respiratory tract: The inhaled particles are trapped in the mucous secretions of the respiratory tract. Cilia propels the particles towards the pharynx. Cough is an important defence mechanism.
• Intestinal tract: Saliva has an inhibitory effect on many microorganisms. Microorganisms that reach the stomach are destroyed by the acidic pH of the stomach. The intestinal flora prevent the growth and colonization of harmful microorganisms.
• Conjunctiva:Lysosyme present in the tears has a bactericidal action. Tears flush away bacteria and dust particles from the eyes.
• The genitourinary tract: Urine flushes away bacteria from the urinary tract. The acidic pH of vaginal secretions and the semen in males has antibacterial substances.

2. Antibacterial substances: Properdin, complement, lysozyme, betalysin, basic polypeptides,(leukins from leukocytes and plakins from platelets. Interferons have antiviral activity.

3. Cellular factors: When an infective agent invades the tissues, phagocytes accumulate at the site and cause an exudative inflammatory reaction. hagocytic action is of 4 types

• Chemotaxis: Phagocytes reach thee site of infection attracted by chemotactic substances.
• Attachment: The infective agent gets attached to the phagocytic membrane.
• Ingestion: Phagocyte engulfs the infective agent(phagosome).
• Intracellular killing:Phagolysosome destroys most bacteria by ingestion. Some bacteria are not killed even after ingestion and they multiply within the phagolysosome and produce disease eg: M.tuberculosis and M.leprae.

4. Inflammation: Inflammation occurs as a result of tissue injury or irritation and is initiated by the entry of pathogens, It is a non specific defence mechanism. Inflammation leads to vasodilation, increased vascular permeability and cellular infiltration. Microorganisms are phagocytosed and destroyed. A fibrin barrier is laid to wall off the site of infection.

5.Fever: A rise in body temperature destroys the microorganisms. Fever stimulates the production of interferon which is helpful in viral infections.

6. Acute phase proteins: After an infection or injury, there is an increase in the plasma concentration of acute phase proteins. These proteins activate the alternate pathway of the complement.

2.Acquired immunity.

The resistance acquired by the individual during life is called acquired immunity.

It is of 2 types:

Active

Passive

1. Active:

• Natural: It is acquires from clinical or subclinical infections. Such immunity is long lasting. Eg people recovering from small pox.
• Artificial : It is produced by vaccination. The vaccines have live, attenuated, killed microorganisms or their antigens or toxoids. Toxoids are immunogenic not toxigenic.

Vaccines are: Live vaccines eg BCG for tuberculosis

                      Killed vaccines eg Hepatitis B vaccine

                      Bacterial products eg Tetanus toxoid for tetanus.

Mechanism: 

Humoral immunity: It is antibody mediated immunity. The plasma cells produce antibodies. The antibodies cause lysis or neutralize the toxins of the antigens.

Cell mediated immunity: It is T- cell mediated immunity. The T cells are important in resistance to chronic bacterial(tuberculosis) and viral(Herpes simplex) infections.

2. Passive immunity:

Passive immunity is induced in the body by preformed antibodies. It is short lasting.

It is of 2 types:

Natural: It is transferred from mother to foetus through transplacental maternal antibodies IgG and to infant through milk.

Artificial: It is through administration of antibodies parenterally.The agents used are hyperimmune sera of animal or human origin, convalescent sera and pooled human gammaglobulin. Hyperimmune sera like Antitetanus serum is prepared by  injecting doses of tetanus toxoid to horses and bleeding them to obtain the serum. Convalescent sera( sera of patients recovering from infectious diseases) is used for the passive immunization against measles and rubella. Pooled human gammaglobulin is used against the passive immunization against viral infections like hepatitis A.

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