Question

multiple choice question
what does the first line of defense against pathogens consist of?

○ leukocytes and macrophages
○ specific defenses
○ external barriers

Explanation:

The first line of defense against pathogens is the innate (nonspecific) defense, which includes external barriers like skin, mucous membranes, etc. Leukocytes and macrophages are part of the second/third lines (cellular/innate - immune cells, or adaptive). Specific defenses are adaptive (third line). So "External barriers" is correct.

Answer:

C. External barriers (assuming the options are ordered as given: first option Leukocytes..., second Specific..., third External; if identifiers are A, B, C: C. External barriers)