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.
In fiction, a foil is a character who contrasts with another character (usually the protagonist) in order to highlight particular qualities of the other character. In some cases, a subplot can be used as a foil to the main plot. This is especially true in the case of metafiction and the  story within a story  motif. The word foil comes from the old practice of backing gems with foil in order to make them shine more brightly.