When the human body is attacked by microbes or pathogens, it defends itself by some mechanisms. There are two types of mechanisms – one which is non specific and the other which is specific to the attack. The non specific mechanism is similar no matter what attacks the body and is again sub divided into external and internal defences.

The external defence mechanism comprises of creating physical and chemical barriers which will prevent the entry of the pathogens. The entry of pathogens into the body is largely prevented by the skin which acts as a protective layer. The hard and insoluble fibrous protein called the keratin in the skin is both water proof and germ proof. Most viruses and bacteria are prevented from entering the body by the skin.

Another external defence factor of the body is the mucous membrane. All the tracts of the human body are lined with the mucous membrane which helps in trapping the microbes and the mucous secreted by the membrane immobilizes these microbes. The microbes trapped in these membranes are either excreted out through the excretory system or thrown out as sputum.

Chemicals secreted by the skin and mucous membranes act as chemical barriers to disease producing microorganisms. The sweat secreted by sebaceous glands makes the skin surface acidic. The acidic environment proves to be adverse for the growth of most microbes, hence destroying them. Saliva also contains a chemical called lysozyme which kills all microbes that are not part of the normal digestive process. Some microbes sustain the chemicals of the saliva and respiratory tract. They successfully reach the stomach where they are killed by hydrochloric acid and proteolytic enzymes present in the gastric juice.

Lysozyme is also present in the tears secreted by the eyes. They help in preventing eye infections. Bile is an alkaline chemical which checks the growth of foreign bacteria on semi digested food. Ears also produce a chemical called cerumen which prevents the entry of insects into the ear. It also contains an effective anti bacterial component.