What term refers to an extreme sensitivity reaction to a normally harmless substance?

The term that refers to an extreme sensitivity reaction to a normally harmless substance is "allergy." Allergies occur when the immune system mistakenly identifies a harmless substance, such as pollen, pet dander, or certain foods, as a threat. This misidentification triggers an immune response, which can result in various symptoms ranging from mild, such as sneezing and itchiness, to severe, such as anaphylaxis.

Understanding allergies is particularly important in a manicure and beauty context, as clients might have sensitivities to certain products like nail polishes, adhesives, or chemicals used in manicure procedures. For professionals in the field, recognizing and accommodating these sensitivities can lead to improved client safety and satisfaction.

The other terms do not pertain to this context. Pathogens refer to microorganisms that can cause disease, infections are the result of pathogens invading the body, and contagions refer to diseases that can be transmitted from one individual to another. Each of these concepts relates to health and illness in a different way, but they do not capture the essence of allergic reactions to harmless substances.