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How do allergies differ from autoimmune diseases?

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immune system disorder False-color scanning electron micrograph of a T cell infected with HIV (human immunodeficiency virus), the agent that causes AIDS (acquired immunodeficiency syndrome).

The primary difference between allergies and autoimmune diseases lies in the target of the immune system's response.

Allergies: These occur when the immune system mounts an inappropriate, hypersensitive response to innocuous foreign substances, known as allergens, such as pollen, dust, or certain foods. The immune system mistakenly identifies these harmless materials as threats, leading to symptoms upon reexposure.12
Autoimmune Diseases: These conditions result from a malfunction where the immune system fails to distinguish between foreign invaders and the body’s own healthy cells. Consequently, immune components attack the body's own normal tissues and organs.3

While these conditions are distinct, they share underlying mechanisms. Immunologists categorize both using the Gell-Coombs system, which identifies four types of hypersensitivity reactions. With the exception of Type I (immediate) hypersensitivity—which is responsible for most common allergies—the mechanisms involved in Types II, III, and IV reactions are observed in both allergic and autoimmune disorders. For example, both can involve inflammation caused by immune complexes or T-cell-mediated tissue damage.21