Which type of hypersensitivity reaction is characterized by immediate drug reactions such as urticaria?

Type I hypersensitivity reactions are characterized by immediate allergic responses, often occurring within minutes of exposure to an allergen or drug. These reactions are mediated by IgE antibodies that bind to allergens and trigger the release of histamine and other inflammatory mediators from mast cells and basophils. This release leads to symptoms such as urticaria (hives), angioedema, and anaphylaxis.

In the context of immediate drug reactions, such as urticaria, this hypersensitivity type is directly involved. For example, certain medications can prompt the immune system to produce IgE antibodies, which then cause mast cell degranulation and the subsequent release of histamine, resulting in skin reactions like hives.

Other hypersensitivity types differ significantly in their mechanisms and timing. Type II hypersensitivity involves antibody-mediated cellular destruction and typically manifests as blood transfusion reactions or certain autoimmune disorders. Type III is characterized by immune complex formation and can result in systemic conditions, such as serum sickness. Type IV hypersensitivity is delayed and mediated by T cells, often taking hours to days to manifest, which is not consistent with the immediate nature of urticaria caused by drug reactions. Thus, the focus on the immediate immune response in Type I hypersensitivity is