Microbiology Practice Exam

Which component is NOT part of the first line of defense?

1. Skin

2. Mucous membranes

3. White blood cells

4. Secretions

The correct answer is: White blood cells

The first line of defense in the immune system consists of physical and chemical barriers that prevent pathogens from entering the body. Skin acts as a formidable barrier, providing a tough exterior that protects underlying tissues from infection. Mucous membranes line various cavities and organs, producing mucus that traps pathogens and prevents them from entering the body. Secretions, such as saliva, tears, and sweat, contain antimicrobial substances that further enhance the body’s defenses against invading microbes. White blood cells, however, are not part of the first line of defense; rather, they play a crucial role in the second line of defense. This latter line consists of immune responses that are activated once pathogens breach the initial barriers. White blood cells actively engage in recognizing and responding to infections, including phagocytosis and producing antibodies, making them integral to the body’s adaptive and innate immune responses, but not to the primary defenses. Therefore, the presence of white blood cells distinguishes the second line of defense from the physical and chemical barriers that compose the first line.