  We proposed an action of neutral fermions interacting with external
electromagnetic fields to construct a 3+1 dimensional topological field theory
as the effective action attained by integrating out the fermionic fields in the
related path integral. These neutral quasiparticles are assumed to emerge from
the collective behavior of the original physical particles and holes
(antiparticles). Although our construction is general it is particularly useful
to formulate effective actions of the time reversal invariant topological
insulators.
