  We study the behavior for $t$ small and positive of $C^{2,1}$ nonnegative
solutions $u(x,t)$ and $v(x,t)$ of the system \[0\leq u_t-\Delta u\leq
v^\lambda\] \[0\leq v_t-\Delta v\leq u^\sigma\] in $\Omega\times (0,1)$, where
$\lambda$ and $\sigma$ are nonnegative constants and $\Omega$ is an open subset
of ${\mathbb R}^n$, $n\ge 1$. We provide optimal conditions on $\lambda$ and
$\sigma$ such that solutions of this system satisfy pointwise bounds in compact
subsets of $\Omega$ as $t\to 0^+$. Our approach relies on new pointwise bounds
for nonlinear heat potentials which are the parabolic analog of similar bounds
for nonlinear Riesz potentials.
