momie

An older middle-aged woman came into our clinic to pick up her husband's Lab results for a routine physical. The clinic doc looked over his batch of Lab reports and then told her that there was a problem.
"We lost the identification information for two patients. We know that one of the reports is for your husband but we do not know which one, and I am afraid that there are bad results on each".
"What results are those" the wife asked nervously.
"One report shows early Alzheimer's Disease, the other is HIV positive. I'm sorry." said the doctor.
"But, can we repeat the tests to see which one is my husband?'
"I checked with Medicare already", replied the doc, "they will not allow us to repeat the same tests."
The woman, now even more upset, asked "But what should I do?"
The doc replied "I asked Medicare that, and they suggested that you drive your husband into the middle of town and leave him there to see what happens."
"Then what?"
"Well, if he finds his way home, don't go to bed with him!"

Firstly; remember that the AHG is incorporated into the gel column.
Cells are added to each column then undergo a specific centrifugation phase that draws cells through the column, but is too 'light' to have a significant affect on free immunoglobulin. So, during the centrifugation phase, red cells come in to contact with AHG, but the AHG is not neutralised, because it is not exposed to free immunoglobulin (only any immunoglobulin that happens to be bound to the red cells).
The same principle applies to the IAT, but with a prior incubation phase (maintaining an 'air gap' between the red cell/plasma mixture and the AHG impregnated gel column) to allow antibody and antigen to bind.