What is the link between the alpha-1 antitrypsin deficiency and the emphysema?

Destruction. Alpha-1 anti-trypsin is a protective chemical in the body that prevents destruction of lung and other tissues from the damage that the infection fighting cells can inflict. When you have a lack of this chemical, the destruction of the infection fighting cells in the body can go unopposed. This is especially true when someone smokes, which attracts those cells to fight off the toxins in the smoke.
Good and Bad. Alpha one is a disease where a good enzyme is missing in the bloodstream. Normally this enzyme prevents bad enzymes from destroying lung tissue. If the level of the good enzyme is low then lung tissue is destroyed resulting in emphysema. There are both mild and severe forms of Alpha one.