Hypersensitivity Reactions

By completing this question set, you will be able to classify any immune-mediated disease into one of the four hypersensitivity types based on its mechanism, mediator, timeline, and pathologic pattern. You will trace the Type I pathway from Th2 sensitization through IgE-mediated mast cell degranulation to anaphylaxis, distinguishing preformed from newly synthesized mediators and early from late-phase responses. You will identify the three cytotoxic mechanisms of Type II hypersensitivity (complement lysis, opsonization, ADCC) and the two non-cytotoxic variants (stimulating and blocking antibodies), and classify diseases accordingly. You will explain how immune complex size and antigen-antibody ratio determine Type III pathogenicity, and distinguish serum sickness from Arthus reaction by mechanism and timeline. You will describe the two subtypes of Type IV hypersensitivity (CD4+ Th1 macrophage activation and CD8+ CTL cytotoxicity) and explain granuloma formation as a consequence of persistent antigen. Finally, you will compare all four types in a diagnostic framework and classify diseases that involve mixed or overlapping mechanisms.