Theoretical predictions from a simple theory for a homogeneous system with respect to some fundamental functional relations between amount of precipitate or extent of turbidity, initial antigen (Ag) respectively antibody (Ab) concentration, and solubility of immune complexes (ICs) are discussed in comparison with experimental results in a heterogeneous system. Experiments were performed with the aim to render possible and intuitive picture of the relationships considering different aspects. It was found that the theory derived from the equilibrium state of the reaction is also able to describe the non-equilibrium state in a qualitatively correct manner. The influence of Ab affinity on precipitation is discussed. Moreover, it is shown that the theory developed by Pauling et al. in the 1940's although too simple in several details, does, however, consider the most important basic principles of more extensive theories developed by other authors. Precisely because of its simplicity, this model may be a useful help in the discussion of precipitate formation on a molecular level.