One of the most common tree nut allergies is a hazelnut allergy. Also known as filbert nuts or cobnuts, hazelnuts have the resemblance of an acorn when they're in the shell. There are two types of hazelnut allergies: A primary food allergy and a secondary food allergy. A primary food allergy is caused by coming into contact with hazelnuts and then the immune system produces antibodies against the hazelnut. When the body does build antibodies against a food after already coming into contact with it, it's called sensitisation. A primary allergy has the ability to trigger a life-threatening allergic reaction, usually causing difficulty breathing (anaphylaxis). A secondary food allergy to hazelnuts is usually caused by an initial allergy to tree pollen which has a similar protein to the protein that is found in hazelnuts. A reaction to two similar but different proteins is called cross-reactivity. Both types of the allergy result in the body reacting by producing antibodies and histamines. Hazelnut allergy symptoms include the following: Runny nose and eyesBlotchy skin or hivesTrouble breathingItchingSwellingBurning in the throat or mouthIf you are allergic to one type of nut, it is likely that you are allergic to other types of nuts. However, if you are allergic to one type of nut, we recommend getting tested for other nut allergies to expand your dietary options and improve your quality of life.