What serves as the body's first line of defense against infection?

The skin serves as the body's first line of defense against infection due to its role as a physical barrier that prevents pathogens, such as bacteria and viruses, from entering the body. It is composed of multiple layers, with the outermost layer providing a tough, impermeable surface that can resist invasion by microorganisms.

Additionally, the skin contains various components that contribute to its protective function, such as antimicrobial peptides and the resident microbiota, which also help inhibit the growth of harmful pathogens. By acting as a barrier and responding to potential threats, the skin plays a critical role in preventing infections from taking hold.

While immune cells, mucous membranes, and blood pressure all contribute to the body's overall defense mechanisms, they do not serve as the initial physical barrier that the skin provides. Immune cells are involved in the immune response after an infection has occurred, mucous membranes help trap pathogens in mucus, and blood pressure is related to circulation rather than direct defense against infection.