Why Do Lymph Nodes Shrink?

I have extra-nodal marginal zone lymphoma which I do not understand this I found out December 24, 2008 I am stage 4 with no symtoms yet I was wondering can anyone tell me why lymph nodes shrink? I had a few in my upper outside arm alos in my breasts and at the bottom of my rib cage. The ones in my arms and breasts have shrunk and I cannot feel them but I can feel the ones at the bottom of the ribs?