[BioC] interaction term contrast in Weaver mutant example of LIMMA manual

[Erika Melissari]

Dear bioconductor list,

I am studing the Weaver experiment example of LIMMA manual and I am in a
maze about the biological meaning of interaction term present among the
contrasts.
I did not manage to understend what kind of differentially expressed gene
this contrast takes-out.
The interaction contrast is:

(P21mt-P11mt)-(P21wt-P11wt)

The first bracket takes-out the differentially expressed genes between the
mutant subjects at two different time points(21 and 11 minutes) and the
second bracket those between the wild-type subject at the same different
time points.
..but what differentially expressed genes takes-out the global interaction
contrast?

In order to evaluate an interaction do I need absolutely the four arrays
in direct comparison done in this example?

Thank you very much for any explaination

Erika