Click image to open full size
Immunology Hypersensitivity Types Cheat Sheet
A printable reference covering Type I, Type II, Type III, and Type IV hypersensitivity mechanisms, mediators, examples, and timing for grades 11-12.
Hypersensitivity reactions are harmful immune responses that damage tissues while trying to defend the body. This cheat sheet helps students compare the four major types by trigger, immune mechanism, timing, and clinical examples. It is useful for connecting immunology vocabulary to real medical conditions such as allergies, transfusion reactions, lupus, and contact dermatitis. Type I reactions are immediate IgE and mast cell reactions, while Type II reactions involve IgG or IgM binding to cells or tissue surfaces. Type III reactions are caused by immune complex deposition, and Type IV reactions are delayed T cell mediated responses. A strong way to remember them is: I = allergy, II = antibody against cells, III = immune complexes, IV = T cells.
Key Facts
- Type I hypersensitivity is IgE mediated and occurs when allergen + IgE on mast cells causes degranulation and release of histamine.
- Type I timing is usually seconds to minutes, with examples including anaphylaxis, allergic rhinitis, asthma, and urticaria.
- Type II hypersensitivity is antibody mediated and occurs when IgG or IgM binds antigens on cells or tissues, leading to complement activation, opsonization, or receptor dysfunction.
- Type II examples include autoimmune hemolytic anemia, transfusion reactions, hemolytic disease of the newborn, Goodpasture syndrome, Graves disease, and myasthenia gravis.
- Type III hypersensitivity occurs when antigen + antibody immune complexes deposit in tissues and activate complement, causing inflammation and neutrophil damage.
- Type III examples include systemic lupus erythematosus, serum sickness, Arthus reaction, and some forms of glomerulonephritis.
- Type IV hypersensitivity is delayed and T cell mediated, with CD4 helper T cells and CD8 cytotoxic T cells causing inflammation or cell killing after 24 to 72 hours.
- Type IV examples include contact dermatitis from poison ivy or nickel, the tuberculin skin test, type 1 diabetes, and transplant rejection.
Vocabulary
- Hypersensitivity
- An exaggerated or misdirected immune response that causes tissue injury instead of only protecting the body.
- IgE
- An antibody class that binds mast cells and basophils and is central to allergic Type I reactions.
- Mast cell
- An immune cell found in tissues that releases histamine and other inflammatory chemicals during allergic reactions.
- Complement
- A group of blood proteins that can be activated by antibodies or immune complexes to promote inflammation, cell lysis, and phagocytosis.
- Immune complex
- A cluster of antigen bound to antibody that can deposit in tissues and trigger inflammation.
- T cell mediated immunity
- An immune response driven by T lymphocytes rather than antibodies, especially important in Type IV hypersensitivity.
Common Mistakes to Avoid
- Calling every allergic reaction Type I is wrong because only IgE and mast cell mediated immediate reactions are Type I.
- Confusing Type II and Type III is common, but Type II antibodies bind fixed cell or tissue antigens while Type III immune complexes float, deposit, and inflame tissues.
- Forgetting the timing of Type IV reactions leads to wrong classification because Type IV is delayed, usually appearing 24 to 72 hours after exposure.
- Assuming all antibody reactions use IgE is wrong because Type II usually uses IgG or IgM, while Type I uses IgE.
- Mixing up Graves disease and myasthenia gravis with Type III is incorrect because both are Type II receptor-targeting antibody diseases.
Practice Questions
- 1 A patient develops hives, wheezing, and low blood pressure 5 minutes after a bee sting. Which hypersensitivity type is most likely, and what antibody is involved?
- 2 A patient receives the wrong blood type and develops red blood cell destruction due to IgM antibodies and complement. Which hypersensitivity type is this?
- 3 A patient develops an itchy rash 48 hours after contact with poison ivy. Which hypersensitivity type is most likely, and which immune cell type is mainly responsible?
- 4 Explain why systemic lupus erythematosus is classified as Type III hypersensitivity rather than Type II hypersensitivity.