Question 1

What is allergen cross-reactivity?

Accepted Answer

Cross-reactivity happens when proteins or chemical shapes in one substance resemble another closely enough that your immune system treats them as the same threat. In contact allergy, chemists group molecules into families (for example, formaldehyde releasers) because those families predict co-reactions.

Question 2

Do I have to avoid every cross-reactor if I am allergic to the main allergen?

Accepted Answer

Not always. Some patients react broadly across a chemical family; others only react to the specific molecule they were tested for. Your dermatologist interprets patch-test grades and may recommend additional tests for finished products.

Question 3

What is co-sensitization?

Accepted Answer

Co-sensitization means two allergies show up together often — sometimes because exposures cluster (nickel and cobalt in jewelry), not because the molecules are identical. It is a statistical pattern, not a guarantee you are allergic to both.

Question 4

How is this different from the ingredient scanner?

Accepted Answer

This tool explains related allergen groups for a known patch-test result. The scanner checks a specific product’s INCI list against your saved profile. Use them together: learn the pattern here, then automate screening in the app.

Question 5

I looked up niacinamide (or retinol / vitamin C). Why isn’t it here?

Accepted Answer

Niacinamide and most “actives” in skincare routines are not classic patch-test cross-reactor families in this database. If your question is “can I layer or combine these ingredients in my routine,” use the Ingredient conflict checker instead. This page is for “I tested positive to allergen X — what other label names might cross-react?”

Patch-test cross-reactor lookup

1 · Your allergen

Cross-reactivity in short

Structural cousins

Co-sensitization

Formaldehyde family

Frequently asked questions