Answer: Jellyfish have stinging cells (cnidoblasts) on their tentacles which inject a toxin into the skin.
The tentacles of a jellyfish carry thousands of stinging cells (cnidoblasts). Each contains a capsule called a nematocyst with a coiled, thread-like tube. When the tentacles touch our skin, the nematocysts discharge their threads, which pierce the skin and inject a toxin. This toxin causes burning, pain, redness, itching and swelling (and in sensitive persons even difficulty in breathing). Hence the body gets irritated on contact with a jellyfish.