We develop the theory of propagation of laser wave in a gas of two-level atoms (with an optical transition frequency $omega^{}_0$) under the condition of inhomogeneous Doppler broadening, considering the self-consistent solution of the Maxwell-Bloch equations in the mean-field approximation and for one-atomic density matrix. The nonlinear effects in the atomic density $n$, caused by the free motion of atoms, are found. These effects distort the lineshape (shift, asymmetry, broadening), but are not associated with atom-atom interaction. Moreover, in the case $nk^{-3}_0<1$ (where $k^{}_0=omega^{}_0/c$) and temperatures $Tgtrsim 300$~K, these quasi-collective effects exceed the well-known influence of the dipole-dipole interatomic interaction (e.g., Lorentz-Lorenz shift) by more than one order of magnitude. It was also found that for some area of parameters, the frequency interval appears, within which the non-trivial self-consistent solution of the Maxwell-Bloch equations is absent at all. Thus, the physical picture of collective effects in a gas medium should be substantially revised.
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