Let $X$ be an $n$-element set. A set-pair system $\mbox{$\cal P$}=\{(A_i,B_i)\}_{1\leq i\leq m}$ is a collection of pairs of disjoint subsets of $X$. It is called skew Bollobás system if $A_i\cap B_j\neq \emptyset$ for all $1\leq i<j \leq m$. The best possible inequality $$ \sum_{i=1}^m \frac{1}{|A_i|+|B_i| \choose |A_i|}\leq n+1. $$ is established along with some more results of similar flavor.