Biology Pages
Kimball's Biology Pages: T Helper Cells
A good site devoted to the T-cell and its role in the immune response. Complete coverage including vocabulary links throughout.
Biology Pages
Kimball's Biology Pages: B Cells and T Cells
A very strong overview of cell mediated immunity and the roles of the different cells within the system.
Other
Hon: Helper T Cell
A very short, but decent, introduction to the function of the memory cells, this page has abundant links to vocabulary help.