9.2.2 Innate Immune System
People have some immunity from birth, ready to fight against invaders immediately. This response is non- specific and general. It has first and second line of defence.
1. First Line of Defence
The external barriers of our body serve as the first line of defence against pathogens. It includes skin and mucous membranes of digestive and respiratory system. The multiple defences including chemical, physical and biological barriers make the first line of defence extremely effective to prevent microbial entry.
2. Second Line of Defence
If pathogens successfully enter the skin or mucous membranes, a second line of defence is activated to combat these foreign invaders. It consists of four nonspecific internal defences. First, the body keeps a constant army of phagocytes, neutrophils and natural killer cells which kill pathogens by phagocytosis. Second, the invasion of bacteria produces inflammation which removes pathogens and initiates healing process. Third, the body develops fever to make the environment less favourable for pathogen growth. Fourth, many protective proteins including complement proteins and interferons are produced which kill bacteria and viruses. These defences are nonspecific because they target a large range of germs rather than specific microbes.
3. Third line of defence/Adaptive (Acquired) Immune System
The third line of defence is also called adaptive or acquired immune system. It consists of immune cells that target specific antigens. The immune cells that play a role in the third line of defence are B- cells and T- cells. The B- cells produce antibodies. The T- cells help identify pathogenic cells and destroy targeted cells.