It is accomplished by providing different types of barriers.
a. Physical barriers These barriers do not allow pathogens and foreign agents to enter the body, e.g., skin, mucous membranes of digestive, respiratory and urinogenital tracts trapping microorganisms.
b. Physiological barriers Sweat, tears, acid in the stomach and saliva prevent microbial growth.
c. Cellular barriers WBCs (polymorphonuclear leukocytes and monocytes, natural killer lymphocytes) and macrophages phagocytose and destroy microbes.
d. Cytokine barriers Interferons produced by virus-infected cells protect noninfected cells from further viral infection.
