hypersensitivity
Meaning of hypersensitivity: The damage comes from the response rather than the substance, as harmless proteins provoke a reaction built for parasites and pathogens.
Definition of hypersensitivity
Hypersensitivity is an immune response out of proportion to the threat, so that the reaction itself causes the harm. The trigger is often something harmless, such as a food, a pollen, a metal or a medicine. Reactions are sorted into four classes according to which arm of the immune system drives them and how quickly they appear, from minutes to several days.
What are the four types?
| Type | What drives it | Typical picture |
|---|---|---|
| Type I, immediate | IgE antibody sitting on mast cells | Hives, swelling, wheeze or a collapse in blood pressure within minutes |
| Type II, against cells | Antibody binding to a target on a cell surface | Destruction of red cells or platelets, sometimes set off by a drug |
| Type III, immune complex | Clumps of antibody and antigen settling in tissue | Fever, rash, joint pain and kidney inflammation days after exposure |
| Type IV, delayed | T cells and the cells they summon | Contact dermatitis from nickel, and many drug rashes appearing after days |
The scheme is old but still useful because the type predicts the timing, the appearance and how a reaction is confirmed. Skin prick testing detects the first type; a patch test left in place for days detects the fourth. A single substance can provoke more than one, which is why a rash appearing a week into a course of antibiotics behaves nothing like the reaction that follows a first injection.
Why is a previous exposure usually needed?
The immune system has to meet a substance before it can overreact to it. That first encounter, called sensitization, produces the antibodies or primed T cells that a later exposure then triggers, which is why a reaction commonly appears to arrive out of nowhere on a medicine taken safely before. The rule has exceptions worth knowing. Something never encountered can resemble something that has been, and cross reaction does the rest. Contact with a related substance in food, cosmetics or the environment can sensitize a person quietly.
Is every reaction to a drug an allergy?
No, and the distinction changes what happens next. Some medicines prod mast cells into releasing their contents directly, with no antibody involved at all, producing flushing, hives and wheeze on a first ever dose. Others cause predictable dose related effects that get labeled as allergy in the record and follow a person for life. Sorting a genuine immune response from these look alikes matters because an incorrect allergy label removes useful treatments for years. A delayed type hypersensitivity response in particular is easy to misattribute to whichever drug was started most recently.
Used in a sentence
The medication was discontinued after the patient developed a hypersensitivity reaction with widespread rash.
Good to know
A hypersensitivity reaction usually requires earlier sensitization to the substance, unlike a directly toxic effect, although reactions can appear on a first exposure through cross reaction or through direct activation of mast cells.
Word origin
Where “hypersensitivity” comes from
Formed from hyper-, Greek for over or above, and sensitivity, from Latin sentire, meaning to feel.