What are the three lines of defense in host defense mechanisms as described in the chapter?

The three lines of defense in host defense mechanisms are: the first line of defense, which includes physical and chemical barriers; the second line of defense, which involves nonspecific protective cells and fluids like inflammation and phagocytosis; and the third line of defense, which is specific and acquired, involving lymphocytes and antibodies.

The first line of defense consists of physical barriers such as the skin and mucous membranes, as well as chemical barriers like stomach acid and enzymes that prevent pathogens from entering the body. The second line of defense includes internal nonspecific responses such as inflammation, phagocytosis, fever, and the action of interferons. These responses act quickly to contain and eliminate pathogens that breach the first line of defense. The third line of defense is specific and acquired, involving the adaptive immune system. It includes the action of lymphocytes (B and T cells) and the production of antibodies, providing targeted responses against specific pathogens and creating immunological memory for faster responses in future encounters.