My background on this comes from a neuroscience course in gradschool. The idea is that molecules and proteins “bind” to receptors which cause a cascade of reactions. The binding depends on the shape of the proteins and receptors.
A bridge like protein is created by removing parts of the spike protein through the cascade. The bridge connects the virus and the cell, and folds into itself to cause the two cells to merge.
https://youtu.be/Xuc9D4LVJdg
I am not sure why it is called spike but I’d guess it’s because it looks like tiny spikes on the virus.
So when what we do with mRNA vaccines is produce the mRNA that encodes this protein, and wrap it in lipids which allow it to enter the cells and the cells manufacture it.
Now, our cells don’t naturally produce the spike protein, so this triggers our immune system to build antibodies which bind onto the proteins and render them useless, or flags for certain parts of our immune system. This results in increased production of certain cells and proteins for this specific protein.
Our immune system has the capacity to create antibodies for and bind onto all sorts of different proteins. This is because it the immune system is allowed to recombine and order its DNA. In order to avoid wrecking havoc at our own body, the immune system passes through filters which filter out proteins that would bind to the naturally occurring proteins in our body.