The body's immune system thinks what he/she is eating or inhaling is a pathogen. The immune system sends white blood cells to attack. This triggers an allergic reaction. For example: I am allergic to asparagus (I'm not joking, I really am) and walnuts. Anytime I eat one of those, my body's immune system thinks it's a pathogen for some reason and attacks and triggers an allergic reaction in me. This makes my breathing hard and makes it really itchy. Good thing it's not that severe so I can eat some if I want, but sometimes allergic reactions could cause hospitalization or death.
The body's immune system thinks what he/she is eating or inhaling is a pathogen. The immune system sends white blood cells to attack. This triggers an allergic reaction.
This question has a two part answer. The first part is that an asteroid hitting the earth and the second part is massive volcanoes erupting around the world.
Some kinds of atoms that are found in DNA must be left out of the models. DNA has so many atoms that the size and cost of a model could be too great. It is impossible to distinguish different kinds of atoms with models.
