However, the structure of lymphatic tissues vary according to the particular type of lymphatic tissue or organ they form a part of - (a), (b) or (c) above, and possibly also it's location in the body-especially in the case of (c) Lymph (Fluid).
The exposition is the portion of a story that introduces important background information to the audience; for example, information about the setting, events occurring before the main plot, characters' back stories, etc.
The device, the gimmick, if you will, or the papers the spies are after... The only thing that really matters is that in the picture the plans, documents or secrets must seem to be of vital importance to the characters. To me, the narrator, they're of no importance whatsoever.