We discuss the structure of an irreversibly adsorbed layer made of neutral, flexible polymer chains (degree of polymerization N) exposed to a solution of mobile chains (degree of polymerization P). We assume that the mobile chains are chemically identical to the adsorbed chains and that their length is shorter (or comparable). According to the value of the bulk volume fraction, phi(b), the mobile chains may, or may not, penetrate into the layer and may, or may not, affect the layer conformation. Using scaling laws, we locate the domains of existence of these different regimes in terms of the variables phi(b), P, and N. In particular, we expect the mobile to penetrate into the layer if the bulk volume fraction phi(b) is greater than a critical value phi(bM) given by phi(bM) congruent to (P/N)(4/7) for P > N-5/12 and by phi(bM) congruent to N--1/3 for P < N-5/12.