According to one site, decreases in hemoglobin concentration and hematocrit were observed in clinical studies with Letairis. Another side effect of Letairis is low red blood count.
I found no information on whether it lowers white blood count. I do know that white blood cells help to fight off infection and, at 400, your uncle's level is much lower than the norm which is 5,000- 10,000.
The most distubing side effects of Letairis are liver damage and birth defects.