  In this article, I study magnetic response of electron wavefunctions in a
commensurate collinear antiferromagnet. I show that, at a special set of
momenta, hidden anti-unitary symmetry protects Kramers degeneracy of Bloch
eigenstates against a magnetic field, pointing transversely to staggered
magnetization. Hence a substantial momentum dependence of the transverse
g-factor in the Zeeman term, turning the latter into a spin-orbit coupling,
that may be present in materials from chromium to borocarbides, cuprates,
pnictides, as well as organic and heavy fermion conductors.
