Question: what in the body happens to give you asthma? I have to do a report for school.
My humble answer: You've come to the right place to find everything you need to know about asthma. The best place to start is by clicking here or, better yet, here. By following these links you should get a good overall understanding of what asthma is and what "triggers" an asthma attack and what an asthma attack is.
No one really understands what causes a person to develop asthma in the first place, but there are theories, such as this one I wrote about.
The airways of most asthmatics are always inflamed (swollen) to some degree. Depending on the severity of this inflammation determines how bad one's asthma is and how sensitive the air passages are to asthma triggers.
When an asthmatic is exposed to his or her asthma triggers, this triggers the asthma response you can read about in the second link above. This ultimately leads to the air passages in your lungs (check out this link) to become increasingly inflamed (swollen) causing them to constrict (become narrow).
When this happens air you breath can enter your lungs, but the narrowed airway traps the air in your lungs (this is called air trapping). Since an asthmatic during an asthma attack has this extra air in his lungs, it feels as though he can't get air in, but the truth is he can't get air out. He then feels like a fish out of water.
Fortunately there are medicines to treat an acute (ongoing) asthma attack like this and even more medicine to prevent an asthma attack. You can read about asthma medicines here.
If you want to read a very thorough writing about what asthma is, you should check out the asthma guidelines I will link to here. Actually, the answer to your question should be in this section.