The four types of hypersensitivity reactions are classified by how the immune system causes damage: Type I (immediate/allergic), Type II (cytotoxic), Type III (immune complex), and Type IV (delayed/T-cell mediated). This classification system was introduced in 1963 by British immunologists Philip Gell and Robert Coombs and remains the standard framework used today. The first three types are driven by antibodies and tend to develop quickly, while Type IV is driven by immune cells called T-cells and takes days to appear.
Type I: Immediate Hypersensitivity
Type I reactions are what most people think of as “allergies.” They occur within minutes of exposure to a trigger like pollen, dust mites, certain foods, medications, or insect venom, and they affect nearly one-third of the global population.
The process starts with sensitization. The first time your body encounters an allergen, your immune system produces a specific type of antibody called IgE. These IgE antibodies attach to the surface of mast cells and basophils, two types of immune cells that sit in your tissues and blood waiting for trouble. Nothing happens yet. But the next time you encounter that same allergen, it binds to the IgE already sitting on those cells, triggering them to release a flood of inflammatory chemicals, most notably histamine. This rapid release is what causes the familiar symptoms: itching, swelling, hives, nasal congestion, watery eyes, or tightening of the airways.
In severe cases, this cascade becomes anaphylaxis, a whole-body reaction that can cause a dangerous drop in blood pressure and airway closure. Epinephrine is the first-line treatment for anaphylaxis because it rapidly counteracts these effects. For everyday allergies, antihistamines work by blocking the histamine that mast cells release. Skin prick testing and blood tests that measure allergen-specific IgE are the standard tools for identifying which substances trigger a Type I response.
Type II: Cytotoxic Hypersensitivity
In Type II reactions, antibodies (IgG or IgM) mistakenly target proteins sitting on the surface of your own cells, essentially marking healthy cells for destruction. Once an antibody binds to a cell surface, the immune system treats that cell as a threat and destroys it through several paths: activating a cascade of proteins that punch holes in the cell membrane, flagging the cell so that immune cells engulf and digest it, or recruiting natural killer cells to trigger the cell’s self-destruct program.
The consequences depend on which cells are targeted. In autoimmune hemolytic anemia, antibodies attack red blood cells, leading to their premature destruction. In immune thrombocytopenia, platelets are the target, which impairs blood clotting. Hemolytic disease of the newborn occurs when a mother’s antibodies cross the placenta and attack her baby’s red blood cells because they carry a different blood type marker. Transfusion reactions, where a patient receives mismatched blood, also fall into this category.
The key distinction from Type I is what the antibodies do. In Type I, IgE antibodies trigger mast cells to release chemicals. In Type II, IgG or IgM antibodies bind directly to cells and cause their destruction.
Type III: Immune Complex Hypersensitivity
Type III reactions involve a different problem: antibodies bind to soluble antigens floating in the bloodstream rather than to antigens fixed on a cell surface. These antigen-antibody clusters, called immune complexes, normally get cleared away by the body. When too many form or the clearing system fails, they accumulate and lodge in tissues, particularly in organs that filter blood heavily, like the kidneys, joints, blood vessels, and lungs.
Once deposited, these immune complexes trigger local inflammation that damages the surrounding tissue. The process typically unfolds over 7 to 10 days after exposure, because that’s how long it takes the immune system to produce enough antibodies to form significant quantities of complexes.
Lupus (systemic lupus erythematosus) is the classic example. In lupus, antibodies target fragments of the body’s own cell nuclei, forming immune complexes that deposit throughout the body and cause widespread inflammation in the kidneys, skin, and joints. Post-streptococcal glomerulonephritis, where kidney inflammation follows a strep infection, is another common example. The Arthus reaction, a localized skin reaction that can occur at the site of a booster vaccine injection, is a more contained version of the same process. Serum sickness, rheumatoid arthritis, and certain types of vasculitis also involve Type III mechanisms.
Type IV: Delayed Hypersensitivity
Type IV is fundamentally different from the other three. It involves no antibodies at all. Instead, T-cells, a branch of the immune system that normally fights off infections inside cells, drive the reaction. Because T-cells take time to mobilize and travel to the affected tissue, symptoms typically appear 48 to 72 hours after exposure, though they can take weeks in some cases. This delay is why Type IV is called “delayed-type hypersensitivity.”
Contact dermatitis is the most familiar example. When a substance like poison ivy oil, nickel, or a chemical in latex penetrates the skin, it binds to skin proteins and creates a new structure the immune system recognizes as foreign. On subsequent exposure, T-cells migrate to the site and release inflammatory signals that damage skin cells, producing the red, itchy, blistering rash most people recognize.
Type IV reactions also play a role in more serious conditions. Transplant rejection happens because T-cells recognize the donor organ’s cells as foreign and attack them. Certain severe drug reactions, including Stevens-Johnson syndrome and toxic epidermal necrolysis (conditions where the skin blisters and peels off in sheets), are T-cell mediated. The tuberculosis skin test deliberately exploits this mechanism: a small amount of protein from the TB bacterium is injected under the skin, and if T-cells that recognize it are present, a firm, raised bump appears at the injection site 48 to 72 hours later.
Patch testing is the primary diagnostic tool for delayed reactions. Small amounts of suspected allergens are applied to the skin under adhesive patches and left in place for 48 hours, then the skin is checked for a reaction. In some cases, intradermal testing with delayed readings at 48 to 72 hours shows higher sensitivity than patch testing alone.
How the Four Types Compare
The simplest way to distinguish the four types is by timing and mechanism:
- Type I: Minutes after exposure. IgE antibodies trigger mast cells to release histamine and other inflammatory chemicals.
- Type II: Hours to days. IgG or IgM antibodies bind to cell surfaces and mark those cells for destruction.
- Type III: Days to weeks (typically 7 to 10 days for initial episodes). Antigen-antibody complexes deposit in tissues and cause local inflammation.
- Type IV: 48 to 72 hours or longer. T-cells, not antibodies, drive the reaction through direct cell damage and inflammatory signaling.
Types I through III all rely on antibodies, which is why they’re sometimes grouped together as “immediate” hypersensitivity, even though Types II and III aren’t truly instant. Type IV stands alone as the only cell-mediated reaction in the classification.
Some medical texts also reference a Type V hypersensitivity, sometimes called stimulatory hypersensitivity. In this variant, antibodies don’t destroy cells but instead overstimulate them. The clearest example is Graves disease, where antibodies mimic the hormone that signals the thyroid, causing it to produce excess thyroid hormone. Type V is not universally accepted as a separate category, and many sources still fold it into Type II since it also involves antibodies binding to cell-surface receptors.