FPGEE for National Association of Boards of Pharmacy (NABP) Practice Exam 2025 - Free NABP Practice Questions and Study Guide

Type IV hypersensitivity is characterized by a delayed reaction, typically occurring hours to days after exposure to the antigen. It is also known as delayed-type hypersensitivity and primarily involves T cells, specifically CD4+ and CD8+ T lymphocytes. When an individual is sensitized to an allergen, subsequent exposure triggers a cell-mediated immune response rather than an immediate antibody-mediated response. This response can lead to inflammation and tissue damage at the site where the antigen is present.

In contrast, Type I hypersensitivity is an immediate reaction mediated by IgE antibodies and associated with allergic responses such as asthma and anaphylactic shock. Type II hypersensitivity involves antibody-mediated destruction of cells, often leading to conditions like hemolytic anemia. Type III hypersensitivity is related to immune complex formation, where antigen-antibody complexes deposit in tissues, causing inflammation and damage.

The key characteristic of Type IV hypersensitivity is its reliance on T cell activation and the delayed appearance of symptoms, distinguishing it from the rapid responses associated with Types I, II, and III.