Chemotherapy Biotherapy Certification ONS Practice Testq

What acts as the first line of defense in the human body?

1. Chemical barriers

2. Natural killer cells

3. Skin and mucous membranes

4. Monocytes

The correct answer is: Skin and mucous membranes

The skin and mucous membranes serve as the first line of defense in the human body due to their role as physical barriers that prevent pathogens from entering the body. The skin acts as a tough and resilient outer covering, which is difficult for microorganisms to penetrate. In addition, the mucous membranes line various cavities and tracts, including the respiratory and gastrointestinal tracts, providing a moist environment that traps pathogens and particles. These barriers not only physically block entry but also contribute to innate immunity through the secretion of substances such as mucus, sweat, and tears, which contain antimicrobial properties. This combination of mechanical protection and biochemical defense creates a formidable obstacle that helps maintain the body’s integrity against infections. In contrast, while chemical barriers, natural killer cells, and monocytes play significant roles in the immune response, they are part of subsequent lines of defense, activated once pathogens breach the initial barriers.