After reading about the immune system for a couple of days, I found out that plants also have an immune system.
They do not have special defender cells to make them immune. They have to use innate immunity provided by the cells. If a pathogen enters the plant, the cell of the plant make antimicrobial products which help neutralize pathogens. Sometimes, the pathogens survive and it triggers a cell of death response. Which causes host cells at the infection to die. According to http://eprints.nottingham.ac.uk/575/1/07icaris_jamie.pdf, the antimicrobial products nee to be tightly controlled and plants have evolved intricate systems to do this.
Plants possess systems that are unique among recognition systems in that they produce responses that are the converse of immune responses.


Pathogens: An agent that causes disease, especially a living microorganism such as a bacterium or fungus.