it is an organ. the main organ in the body. I wish your question was a little more clear, like multiple choice would be very helpful to what my choices are.
Your answer would be The microorganism was unicellular and had a well-defined nucleus.
Bacteria and plants both have cell wall and this is the main similarity !
in accurate all bacteria, cell wall may not be present !
Swelling and pain is mainly due to the expansion of local blood vessles to make them more permeable to white blood cells
hope that helps