Describe innate immunity.

(N/A) Innate immunity is a non-specific type of defense that is present at the time of birth. This is accomplished by providing different types of barriers to the entry of foreign agents into our body. Innate immunity consists of four types of barriers:
$(i)$ Physical barriers: Skin on our body is the main barrier which prevents the entry of micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal, and urogenital tracts also helps in trapping microbes entering our body.
$(ii)$ Physiological barriers: Acid in the stomach, saliva in the mouth, and tears from the eyes all prevent microbial growth.
$(iii)$ Cellular barriers: Certain types of leukocytes $(WBC)$ of our body like polymorpho-nuclear leukocytes $(PMNL-neutrophils)$, monocytes, and natural killer cells (a type of lymphocyte) in the blood, as well as macrophages in tissues, can phagocytose and destroy microbes.
$(iv)$ Cytokine barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.