  The scattering of fermions from the abelian string arising during the phase
transition $SO(10) \rightarrow SU(5) \times Z_2$ induced by the Higgs in the
126 representation is studied. Elastic cross-sections and baryon number
violating cross-sections due to the coupling to gauge fields in the core of the
string are computed by both a first quantised method and a perturbative second
quantised method. The elastic cross-sections are found to be Aharonov-Bohm
type. However, there is a marked asymmetry between the scattering
cross-sections for left and right handed fields. The catalysis cross-sections
are small, depending on the grand unified scale. If cosmic strings were
observed our results could help tie down the underlying gauge group.
