“…This, of course, pleased us immensely. Later, our simple equation for a quantitative theory was shown to be a special case of a more general theory (16) and this, in turn, was found to be a special case of a still more general theory requiring calculations by a computer (17). This was progress, but for immunologists needing simple, quantitative explanations and calculations our formulation and equation remained practical and useful in many, but not all, precipitating systems.…”