You have to keep in mind that active/inactive states of a receptor represent two different conformations of the receptor, sometimes additionally accompanied by different oligomerization states (e.g. dimerization of tyrosine kinase containing receptors). Agonist binding stabilizes the active state, antagonist binding the inactive state.

Depending where an antibody binds, it can have no effect on activity, simulate ligand binding or compete with ligand binding, allosterically alter the conformation to either stabilize the active or the inactive state, sterically hinder oligomerization or by crosslinking receptors through its two antigen binding sites, promote oligomerization.

It is an important question.   I have previously found a similar effect in Hg study (enzyme inhibition and activation: please see file; Thiol-type BIN).   Catalytic center may be changed by 2-mercaptoethanol (ME), and addition of Hg has influenced the chemical structure of catalytic center.  

Similarly, the binding center of IgG may be changed by some molecules or metal atoms.   

Therefore, ELISA can not be a quantitative assay.

You have to keep in mind that active/inactive states of a receptor represent two different conformations of the receptor, sometimes additionally accompanied by different oligomerization states (e.g. dimerization of tyrosine kinase containing receptors). Agonist binding stabilizes the active state, antagonist binding the inactive state.

Depending where an antibody binds, it can have no effect on activity, simulate ligand binding or compete with ligand binding, allosterically alter the conformation to either stabilize the active or the inactive state, sterically hinder oligomerization or by crosslinking receptors through its two antigen binding sites, promote oligomerization.