Answer:

★ Bacteria such as Rhizobium, Azotobacter

★ Memory T cells are necessary for protective immunity against invading pathogens, especially under conditions of immunosuppression. However, their presence also threatens transplant survival, making transplantation a great challenge.

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Two microorganisms involved in nitrogen fixation are Rhizobium and Azotobacter.

• Memory cells are a type of immune cells that play a crucial role in the immune system's ability to remember and respond to previously encountered pathogens. The significance of memory cells lies in their ability to provide long-term immunity against specific pathogens. When the body is exposed to a pathogen for the first time, the immune system mounts a response and produces memory cells specific to that pathogen. These memory cells "remember" the pathogen and remain in the body even after the infection is cleared. If the same pathogen is encountered again in the future, the memory cells quickly recognize it and mount a rapid and efficient immune response, leading to faster clearance of the pathogen. This is the basis of acquired immunity and vaccination, as memory cells provide long-lasting protection against specific diseases.