  A set of fluid equations, taking into account the spin properties of the
electrons and positrons in a magnetoplasma, are derived. The
magnetohydrodynamic limit of the pair plasma is investigated. It is shown that
the microscopic spin properties of the electrons and positrons can lead to
interesting macroscopic and collective effects in strongly magnetized plasmas.
In particular, it is found that new Alfvenic solitary structures, governed by a
modified Korteweg-de Vries equation, are allowed in such plasmas. These
solitary structures vanish if the quantum spin effects are neglected. Our
results should be of relevance for astrophysical plasmas, e.g. in pulsar
magnetospheres.
