Which type of organism is primarily responsible for hypersensitivity reactions?

Hypersensitivity reactions are primarily mediated by allergens, which are environmental substances that provoke an exaggerated immune response. These responses can occur in various forms, including allergic rhinitis, asthma, and anaphylaxis, and they involve the immune system mistakenly identifying a normally harmless substance as a threat.

Allergens can include a wide range of materials, such as pollen, dust mites, pet dander, certain foods, and insect stings. When people who are sensitized to these allergens are exposed to them again, their immune systems react by releasing various chemicals, including histamines, leading to the symptoms of an allergic reaction.

Bacteria, viruses, and parasites do not inherently cause hypersensitivity reactions in the same way that allergens do. Instead, these organisms tend to cause infections and can provoke immune responses, but these are typically different from the hypersensitivity reactions associated with allergens.