Which organisms transform nitrogen to a form that is useful to plant?

bacteria

Explanation:

Bacteria helps in a process called ammonification where ammonia is oxidized to nitrites and then to nitrates and these nitrates are the most usable form for plants.  

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.