CBSE Class 10 Answered

Organisms employ many types of defence to stop the the invasion of pathogens. Means of defence can be categorised into first and second lines of defence, with the first line usually having direct contact with the external environment.

First Lines of Defence:

These include the following:

• Skin is an excellent line of defence because it provides an almost impenetrable biological barrier protecting the internal environment.
• Lysozyme is an enzyme found in tears and saliva that has powerful digestive capabilities, and can break down foreign agents to a harmless status before they enter the body.
• The clotting of blood near open wounds prevents an open space for antigens to easily enter the organism by coagulating the blood.
• Mucus and cilia found in the nose and throat can catch foreign agents entering these open cavities then sweep them outside via coughing, sneezing and vomiting.
• The cell wall of plants consists of fibrous proteins which provide a barrier to potential parasites (antigens).

If these first lines of defence fail, then there are further defences found within the body to ensure that the foreign agent is eliminated.

Second lines of defence :

Second lines of defence deal with antigens that have bypassed the first lines of defence and still remain a threat to the infected organism. These include the action of interferons and white blood cells by using several tactics like phagocytosis to eliminate the foreign antigen.