An irritant is a chemical that causes reversible inflammation. Irritants can be eye, skin, or lung irritants, or a combination of these. Examples of symptoms of irritation include itching, discomfort, red or watery eyes, sneezing, contact (irritation) dermatitis, coughing, and can induce asthma attacks in those with asthma.
An allergen is a usually harmless substance that triggers an immune system response. Examples of allergic reactions include hives, itching, discomfort, red or watery eyes, sneezing, contact (allergic) dermatitis, can induce asthma attacks, and anaphylactic reactions.
Notably there is overlap in symptoms of irritant and allergic reactions. In addition, both can range from mild to severe in terms of symptoms. Finally, some things can be both an irritant and an allergen, such as mould (mold).