The body contains a number of digestive enzymes known as
proteases which break down proteins into their component amino-
acids. These are on the whole confined to the digestive tract
and other places where they are needed, but they do to some
extent get through into the blood plasma, where they tend to
break down the tissues of the body indiscriminately. This
tendency is counteracted by the presence in the plasma of a
protein which functions as a protease inhibitor. This exists in
a large number of forms controlled by a set of alleles, and
having different degrees of inhibitory activity. There is one
with extremely low activity, and there also probably exists an
amorph gene which produces no activity at all.() Individuals
with two low-activity or amorph genes for the protease inhibitor
(Pi) system suffer from one or more diseases characterized by
protein breakdown. They invariably have some degree of pulmonary
emphysema, with breakdown and running together of the alveoli,
the little air sacs of the lungs. They