The thinking is that CDs help stabilize the contact between MHCs and the T cell receptors HIV invades cells expressing CD4, and therefore selectively destroys Th cells. The Th cells release cytokines, thus activating other cells. B cells are very efficient at presenting epitopes because their antibodies make them efficient at internalizing antigens. Macrophages are less specific. The complex is transported to the surface.
Tc cells release cytotoxins to kill the cell, and move on.
B cells become fully mature plasma cells. Tc cells dump cytotoxins. Plasma cells and memory cells result. They are codominant - that is, your paternal and maternal copies are both expressed, making the individual's population of MHCs even more difficult to match. Graft rejection is a byproduct of this polymorphism.