Allergy vs. Toxicity: A Mechanistic Overview
An interactive guide to the critical differences between toxic and immune-mediated adverse reactions.
Reaction Pathways: A Flowchart
Toxic Reaction
This is a direct chemical effect. The severity is proportional to the dose and is not dependent on the host’s immune system. The mechanism is related to the substance’s inherent pharmacological or toxicological properties.
Allergic Reaction (Anaphylaxis)
A true allergic reaction requires two stages. First, the immune system is ‘primed’. On re-exposure, IgE antibodies trigger a massive release of histamine and other mediators, causing the systemic symptoms of anaphylaxis.
Anaphylactoid Reaction
This reaction mimics anaphylaxis but bypasses the IgE sensitisation step. The substance directly triggers mast cells to release their contents. The clinical presentation is identical to anaphylaxis, but the mechanism is pharmacological, not immunological.
Dose-Response vs. Immune Response
This graph illustrates the fundamental difference between a toxic and an allergic reaction. Notice the red line, representing a toxic reaction, shows a graded response: as the dose increases, the severity of the reaction increases proportionally. In contrast, the blue line, representing an allergic reaction, shows a threshold effect. No reaction occurs until a sensitised individual is exposed to a small trigger dose, which then causes a maximal, severe reaction that does not increase with higher doses.