The stinging structures on the tentacles are called nematocysts , which are spring-loaded venom glands that contain small, hollow, sharp tubes. When human skin comes into contact with a jellyfish, the poison is injected into the outermost layers of skin, causing the toxic reaction of a typical sting . The rash is usually limited to the area of skin that came into direct contact with the jellyfish. The lesions can last for days to weeks. Sometimes, an allergic reaction can occur in response to the initial sting, which can lead to swelling and further inflammation of the skin.
Stings involving the eyes result in local inflammation, redness, swelling and tearing.
Severe stings to the eyes can lead to abrasions, which can scar and potentially harm eyesight. One type of jellyfish, called the thimble jellyfish, can release eggs that can lodge within swimwear. This occurs on the skin under the swimwear and usually develops over the first 24 hours and lasts for up to 5 days. Rarely, severe jellyfish stings can cause weakness, headaches, vomiting, muscle spasms, fevers and chills, and difficulty with breathing, and even shock. Stings from the box jellyfish of Australia have been known to cause sudden death.