  The tree-level two-point amplitudes for the transitions $jf \to j^{\, \prime}
f^{\, \prime}$, where $f$ is a fermion and $j$ is a generalized current, in a
constant uniform magnetic field of an arbitrary strength and in charged fermion
plasma, for the $jf$ interaction vertices of the scalar, pseudoscalar, vector
and axial-vector types have been investigated. The particular cases of a very
strong magnetic field, and of the coherent scattering off the real fermions
without change of their states (the "forward" scattering) have been analysed.
