  Let $X$ be a Fano manifold of Picard number one. We establish a lower bound
for the second Chern class of $X$ in terms of its index and degree. As an
application, if $Y$ is a $n$-dimensional Fano manifold with $-K_Y=(n-3)H$ for
some ample divisor $H$, we prove that $h^0(Y,H)\geq n-2$. Moreover, we show
that the rational map defined by $\vert mH\vert$ is birational for $m\geq 5$,
and the linear system $\vert mH\vert$ is basepoint free for $m\geq 7$. As a
by-product, the pluri-anti-canonical systems of singular weak Fano varieties of
dimension at most $4$ are also investigated.
