Which of the following are part of the body's first line of defense against pathogens?

A. white blood cells

B. red blood cells

C. lymph nodes

D. mucous membranes

Pathogens are disease - causing organisms which include bacteria, viruses, and toxins. The first line of defense against these microorganisms include the skin, tears, mucus, stomach acid, urine flow, and others. From the given choices, I would go with the fourth one, mucous membranes.