What symptom is indicative of a Type 4 hypersensitivity reaction?

Type 4 hypersensitivity reactions, also known as delayed-type hypersensitivity, are mediated by T lymphocytes rather than antibodies. They typically occur 1 to 3 days after exposure to an antigen. A classic example of this is the skin rash that occurs after contact with poison ivy. In this scenario, the urushiol oil found in poison ivy triggers an immune response involving T cells that leads to symptoms like localized inflammation and rash.

The reaction occurs through a process where sensitized T cells recognize the antigens on the skin, leading to an inflammatory response that manifests as a rash, redness, and itching. This distinguishes it from other hypersensitivity types, where the immune mechanisms and timeframe of reactions differ. This characteristic feature makes the rash from poison ivy a prime representation of a Type 4 hypersensitivity reaction, as it highlights the delayed response and reliance on cell-mediated immunity.