Crossing Symmetry

The scattering amplitude is invariant under replacing an incoming particle of momentum p with its outgoing anti-particle with momentum -p, including a factor -1 if it is a fermion line. Hence, the new amplitude is given by $\mathcal{M}(k_1,...,k_n\to p_1,...,p_m)=(-1{)}^f\mathcal{M}(k_2,...,k_n\to p_1,...,p_m,-k_1)$ where f=1 if the particle is a fermion (zero otherwise).