The Human Body Is Constantly Challenged by Pathogens in the Environment

The human body is constantly challenged by pathogens in the environment. It is not infected by every pathogen it encounters because the microbial flora

A) engulf the pathogenic bacteria.
B) produce conditions at the microenvironmental level that block colonization.
C) prime our immune system.
D) activate and support the action of antigen-presenting cells, cytokines, and cell-mediated immunity.