Which acts as both a physical and chemical barrier against pathogens?

blood

lymph

mucus

skin

They are mucus and skin and blood (white blood cells)
skin: microorganisms living all over your skin can't get through unless it's broken.
Tears, mucus and saliva contains enzymes that can break down a cell walls of many bacterias. those that are not killed immediately are trapped in mucus and swallowed. (grosss)
white blood cells (WBC) can find, kill and ingest pathogens seeking an entrance in the body.
so instead of lymphs the ans would be all three of them
but as the best definite ans it would be the skin.

Is this used on a test? ?

Answer active artificial immunization;