  We derive and analyze an effective quantum Boltzmann equation in the kinetic
regime for the interactions of four distinguishable types of fermionic
spin-$\frac{1}{2}$ particles, starting from a general quantum field
Hamiltonian. Each particle type is described by a time-dependent, $2 \times 2$
spin-density ("Wigner") matrix. We show that density and energy conservation
laws as well as the H-theorem hold, and enumerate additional conservation laws
depending on the interaction. The conserved quantities characterize the $t \to
\infty$ thermal (Fermi-Dirac) equilibrium state. We illustrate the approach to
equilibrium by numerical simulations in the isotropic three-dimensional
setting.
