Let's take each image in turn. The 3 screen captures shown here are taken directly from the animation.
The first step in becoming allergic to a substance is called sensitization. The "antigen" is whatever substance will be that triggers a future histamine reaction (e.g. nuts, cat dander, pollen, bee stings, dust, mold, penicillin).
So, all we're seeing in this sequence is that the irritating substance is being taken up by a specialized cell, whose job it is to translate that irritant into a signal that other cells will understand is NOT GOOD. So in the top image, the green circle with the 4 triangular points within the darker purple bubble-the irritant-is being translated into two objects that look like cattails. The cattails are called receptors, and are what the other cells will recognize as something bad.
Think of it like translating English into another language so that someone will understand what is being said.