Which of the following is a physical barrier that provides innate immunity against invading pathogens?

The correct option is C Mucus coating of the epithelium lining the respiratory tract
Immunity is the ability of our body to protect against pathogens such as bacteria, viruses, protozoans and other toxins.

Immunity in an organism can be of two types, innate or natural immunity that is inherited since birth and acquired immunity is acquired during the lifetime and is pathogen specific.

Innate immunity is non-specific in nature and is directed towards any foreign substance that invades the body. It acts as the first line of defence and includes various barriers that try to eliminate the pathogens at the point of entry. It includes physical barriers, physiological barriers, cellular barriers and cytokine barriers.

Skin is the primary physical barrier which prevents the entry of pathogens into the body. Apart from skin, the mucus coating of the epithelium lining the respiratory tract, gastrointestinal tract and urogenital tract also acts as a physical barrier which traps foreign bodies that enter these regions.

The mucus coating in the respiratory tract is a physical barrier, which along with the nasal hair, effectively traps dust and foreign particles from the inhaled air and allows clean air to enter the trachea (windpipe).

Saliva in the mouth contains many antimicrobial proteins which helps in killing microbes that enter the mouth via food. For example, lysozyme present in the saliva can break the cell wall of bacterial cells and kill them. Lysozyme is also present in tears and plays a similar role.

Hydrochloric acid is secreted by the cells in the stomach and helps in killing microbes that enter the stomach along with food. The saliva, tears and HCl in the stomach are examples of physiological barriers that provide innate immunity.

Figure: Barriers of innate immunity