Immunity and Immune System

Animals encounter many potentially dangerous microbes in air, water and food. So they have involved defense mechanisms against the disease causing germs Immunity is a defense mechanism without which we will fall a prey to all parasitic microorganisms.

Definition

Immunity is defined as the ability to eliminate complexes of protein molecules differing in structure from those present in the healthy animals.

There are two kinds of defence mechanism against microbes

a) nonspecific defence mechanism

b) specific defence mechanism.

Non-specific defence mechanism

This is common for most type of infection. It controls infection by blocking the entry of pathogens into the body or by destroying the microbes through means other than antibodies.

The non-specific defence mechanism is further of two types

a) External defence or first line of defence

b) Internal defence or second line of defence

External Defence

The external defence comprises physical and chemical barriers to the entry of pathogens into the body.

Physical barriers

Skin

The outer layer of the skin is fully keratinized and is waterproof and germproof. It successfully prevents the entry of viruses and bacteria.

Mucuous Membranes

The mucuous membrane lining the digestive, respiratory and urinogenitaltracts secrete mucus which traps the micro organisms and immobilises them. This is then eliminated with the sputum or removed with the faeces

Secretions of tears and movements of eyelids flush out the microorganisms settling on the eyes.

Chemical Barriers

The skin and mucuous membranes secrete certain chemical which dispose of the pathogens.

Skin Secretions

The oil and sweat secreted by the sebaceous and the sweat glands contain some fatty acids and lactic acid which make the surface of the skin acidic with a pH of 3-5. This prevents micro organisms from growing on the skin. The skin harbours some friendly bacteria also which release acids and other metabolic wastes that check the growth of microbes. Lysozyme, an enzyme present in sweat, kills many bacteria by destroying their cell wall.

Saliva

Saliva also contains lysozyme which kills micro organisms that are not the normal inhabitants of the buccal cavity.

Bacteria which escape the action of saliva, when they reach the stomach, are killed by the hydrochloric acid and the proteolytic enzymes of the gastric juice.

Tear

Tears, a slightly saline fluid secreted by the lachrymal gland over the eyes, also contain lysozyme which prevents eye infections. Frequent washing of eyes reduces the disinfecting power of the lachrymal secretions. Tears also wash off the chemical irritants of polluted air, from the eyes.

Skin and mucous membranes may fail to keep out the invaders. Some parasites enter through the skin e.g., hook worm, or through wounds and swellings of sweat glands and hair follicles. It is therefore very essential to have a second line of defence for controlling the parasites that have entered the body by breaking through the first line of defence.