For some people, summer comes with a risk of many itchy nuisances, including poison ivy. This video from the Centers for Disease Control and Prevention explains how the oils from this plant cause the skin reaction they’re known for.
An oil called urushiol is the secret to the effects of poisonous plants. As the video illustrates, the oil allows chemicals to sneak into the skin, where they bind to proteins in the skin. An important type of immune cell the monitors for foreign substances is alerted to such a situation, and after a series of immunological events that are described in more detail in the video, the resulting inflammation makes us itch.
Check out this fact sheet about poison ivy, which includes tips on protecting yourself.