The term for an unpredictable reaction to a substance is an allergy or hypersensitivity. Allergies occur when the immune system reacts excessively to typically harmless substances, known as allergens. This can result in various symptoms, ranging from mild (sneezing, itching) to severe (anaphylaxis). The unpredictability of allergic reactions makes it essential for individuals with known allergies to avoid their triggers, as even minimal exposure can lead to significant health issues.