What characterizes Type IV hypersensitivity?

• A. It is immediate and involves IgE antibodies
• B. It is mediated by T-cells and occurs later
• C. It involves immune complexes
• D. It is an allergic reaction

Answer: (B)

Type IV hypersensitivity is characterized primarily by its reliance on T-cells rather than antibodies. This type of hypersensitivity is also known as delayed-type hypersensitivity because it typically manifests several hours to days after exposure to the antigen. Unlike other types of hypersensitivity that involve antibodies—such as IgE in Type I or immune complexes in Type III—Type IV responses are orchestrated by sensitized T-lymphocytes.

When the body is exposed to a specific antigen, T-helper 1 cells can become activated and release cytokines, which in turn recruit and activate other immune cells, including macrophages. This response is essential in protecting against intracellular pathogens and plays a significant role in graft rejection, contact dermatitis, and other autoimmune conditions.

In context, it is important to differentiate Type IV from other hypersensitivity types. For example, immediate hypersensitivity reactions involve IgE antibodies and occur rapidly upon re-exposure to an allergen. Immune complexes are the hallmark of Type III hypersensitivity, and allergic reactions can encompass various types including Type I, which is mediated by IgE antibodies.

Thus, the defining characteristics of Type IV hypersensitivity revolve around T-cell mediation and delayed onset, making the choice regarding T-cells and their temporal response the accurate