There is external and internal so I'll list th external. The most important is skin then mucus, sweat or secretion, hair, nails and tears which are all part of the integumentary. Some internal defenses are white blood cells and T cells.
Answer:
Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism,
Ice, water, and steam. :)
hope my answer helped :)