To which category do saliva in the mouth, tears from eyes, and stomach acid belong as protective barriers?

Physiological barriers

The correct answer is Option (3)- Physiological barriers.

Innate immunity is non-specific type of defence, that is present at the time of birth. This is accomplished by providing different types of barriers to the entry of the foreign agents into our body. Innate immunity consist of four types of barriers. These are —

Physical barriers : Skin on our body is the main barrier which prevents entry of the micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.

Physiological barriers : Acid in the stomach, saliva in the mouth, tears from eyes–all prevent microbial growth.

Cellular barriers : Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.

Cytokine barriers : Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.