Why cells in a eukaryaotic organism tend to repel each other?

Hai Divya Pandey,
Most of the cell membrane is made of lipid bilayer and in addition to this they have charges based on the type of proteins.
This makes them attrcact and repell each other.
For example:
1.The F+ and F- bacteria attract each other for conjugation forming an exchage tube to exchange the DNA.(Prokaryotic)
2.Fungi (penicillium notatum) secretes antibiotics to repell the bacteria awaay from them (penicillin).(Prokaryotic)
3.This is an comon phenomenon which wee even observe n human body.
RBC of other blood groups are rejected by forming clumping mechanism.(Eukaryotic)
4.Immune responses against foreign bacteria and microorgansims.(Eukaryotic)