Throughout the night, the human body is exposed to various molds, bacteria, and viruses. yet, in the morning, the human body is left unharmed. what is the human body's first defense against infection from these threats?

a. the skin produces cells that both detect and kill antigens.
b. the skin sheds old skin hourly to ensure newly regenerated skin.
c. the immune system produces t-killer and t- cells at night.
d. the immune system overly produces antibodies as one sleeps.