A mushroom is the reproductive structure produced by some fungi. It is somewhat like the fruit of a plant, except that the "seeds" it produces are in fact millions of microscopic spores that form in the gills or pores underneath the mushroom's cap.Mushrooms are rich in antioxidants like selenium, vitamin C and choline. Along with antioxidants, mushrooms also contain protein, B-complex vitamins, and fiber. All these nutrients together help prevent cell and tissue damage and thus support our immune system.