Re: Where do B cells and Tcells Clone?

First, let’s get the terminology in order. Cells don’t “clone”. That’s a 
word used as a verb in molecular biology, for example you can clone a gene 
(or a person I suppose).  B and T cells can undergo what is called “clonal 
expansion” in response to antigen, meaning that antigen selects one of the 
rare cells (~1/10,000) that has a receptor that can bind to it. The B or T 
cell divides many times, forming a clone of cells all derived from a 
single precursor. B and T cells can proliferate in lymphoid tissues such 
as lymph nodes, spleen, Peyer's patches, and tonsils as well as in non-
lymphoid tissues. 
The “germinal centers” are special structures where the B cells not only 
proliferate but also undergo two other processes. 1. Isotype switching, 
which means they start out making IgM, but then may switch to make any of 
the other antibody classes, IgG, IgA, or IgE. 2. Somatic hypermutation, 
which can change the specificity of the antibody, making it more efficient 
at reacting with the antigen.  T cells don’t do either of these things, so 
there is no structure analogous to the germinal center for T cell clonal 
expansion.
For further information, you may find Kuby’s Immunology textbook helpful. 
http://www.whfreeman.com/kub
y/index.htm