  The helicity of a Dirac neutrino with mass $m$ evolves under the influence of
a $B$-field because it has a magnetic dipole moment proportional to $m$.
Moreover, it was recently shown that a polarized or anisotropic medium
engenders the same effect for both Dirac and Majorana neutrinos. Because a
$B$-field polarizes a background medium, it instigates helicity oscillations
even for Majorana neutrinos unless the medium is symmetric between matter and
antimatter. Motivated by these observations, we review the impact of a
$B$-field and of an anisotropic or polarized medium on helicity oscillations
for Dirac and Majorana neutrinos from the common perspective of in-medium
dispersion.
