Which type of hypersensitivity reaction is represented by the acronym ACID?

The acronym ACID corresponds specifically to Type IV hypersensitivity, also known as delayed or cell-mediated hypersensitivity. This type of reaction involves T lymphocytes rather than antibodies and typically takes hours to days to develop after exposure to the allergen or antigen.

The term "ACID" can be used as a mnemonic to recall the characteristics of Type IV hypersensitivity: A for Allergy (which is not part of Type IV reactions), C for Cell-mediated response, I for Immunological basis, and D for Delayed onset. This reinforces the key feature of Type IV hypersensitivity where the response is mediated by T cells that are activated upon antigen exposure and require time to mount a response, hence the “delayed” nature.

In contrast, Type I reactions (anaphylactic) involve immediate hypersensitivity that is mediated by IgE antibodies and can trigger rapid allergic responses. Type II (cytotoxic) reactions involve IgG or IgM antibodies that target cells and lead to cell destruction, while Type III involves immune complex formation that can lead to inflammation and tissue damage. Each of these types has distinctive mechanisms and time frames, making ACID specifically applicable to Type IV.