After recalling our definition of Auslander-Reiten systems, which generalize almost split sequences of $kG$-modules in terms of idempotents in a symmetric interior $G$-algebra, we give sufficient conditions for the Auslander-Reiten system ending in a given idempotent $i$ to be a pullback from the tensor product $i\otimes \mathcal {L}_k$, where $\mathcal {L}_k$ denotes the Auslander-Reiten system ending in the identity element of the trivial interior $G$-algebra $k$. Our approach and results are different from those of Auslander and Carlson for almost split sequences (J. Alg. 103), and they apply to the significant where $i$ is a source of a block algebra ; in this case we give an explicit generator to the socle of the bimodule relevant for constucting the Auslander-Reiten system.