A theoretical model for investigating physical phenomena underlying im
mune complex formation was developed based on the statistical mechanic
al theory of associating fluids that identifies each molecule as a har
d sphere with a nested point charge and vector dipole. The interaction
between binding molecules (epitope- paratope binding) is represented
as a cone truncated by two concentric spheres in which the potential e
nergy is a modified square well with respect to particle separation an
d a square well with respect to mutual molecular orientation. Equilibr
ium binding results predicted by the model show good agreement with re
sults obtained experimentally for a model system containing a single a
ntigen and a single monoclonal antibody [bovine serum albumin (BSA)-an
ti-BSA antibody]. Moreover values obtained for the system isothermal c
ompressibility and the second viral coefficient by both the model and
light scattering experiments also show good agreement with one another
.