Immune system. Conceptual computer artwork of the immune system protecting the body against invading pathogens. The skin is the body's first line of defence against infection. It is a mechanical barrier that prevents most pathogens from entering the body and is part of the innate immune system, which also includes antimicrobial enzymes secreted by the mucous membranes. If pathogens do enter the body they are recognised by the adaptive immune system. This consists of white blood cells and chemicals that detect and destroy the pathogens. It includes antibodies, B cells and T cells.