|MadSci Network: Immunology|
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
Try the links in the MadSci Library for more information on Immunology.