  In this work a three-states cellular automaton proposed to describe part of a
biological immune system is revisited. We obtain the dynamic critical exponent
$z$ of the model by means of a recent technique that mixes different initial
conditions. Moreover, by using two distinct approaches, we have also calculated
the global persistence exponent $\theta_{g}$, related to the probability that
the order parameter of the model does not change its sign up to time $t$
[$P(t)\propto t^{-\theta_{g}}$].

Statistical Mechanics