Yes, both real and artificial trees can cause allergies because they have dust and mold on them. Dust and mold are a few of the main causes of allergic reactions in humans. Real Christmas trees are cut months before we see them on the lots. They are wrapped and stacked in close and sometimes damp areas where they can pick up mold spores.
Pollen on Christmas trees is usually not a problem for allergy suffers. Most of the type of trees seen on Christmas tree lots pollinate in the Spring. If you cut down your own tree, make sure it is not a type that pollinates in November and December. Even Christmas trees that are cut fresh in the woods can have mold growing on them or have pollen from other types of trees trapped on their branches. Read about ways to prevent Christmas tree allergies.