Each spring and fall, many of us find ourselves experiencing sneezing and watery eyes -- the result of allergies to plants.

Each spring and fall, many of us find ourselves experiencing sneezing and watery eyes -- the result of allergies to plants. What are allergies and why do we experience symptoms only during specific periods of the year? Do all plants produce substances that we humans are allergic to? Are all regions across the globe similar when it comes to the production of allergens? Why? What are some common plant species that humans are allergic to?