Although a number of different types of boson-fermion interactions are possible in the interacting boson-fermion model, empirical results have shown that the exchange term is of cardinal importance. A recently constructed similarity transformation which reexpresses the Dyson boson images of the single-j shell fermion operators in terms of seniority bosons is used to confirm that the origin of the exchange term in spherical nuclei is linked to the quadrupole pairing interaction between identical nucleons. The relative importance of the exchange and direct boson-fermion interactions is also discussed and it is indicated that the direct term is essential for a complete description of that excited state which carries the j value of the shell.