PBS
Pbs Teachers: Surviving Aids: Fighting Back (Hot Science)
Explore the body's immune system to understand how a virus enters the body and how the human body fights against the viruses like the mumps virus and HIV.
PBS
Pbs: The Immune System's Battle Against Disease
A "just for fun" Shockwave game that allows you to control an animated macrophage against an invading virus. You must figure out how the let the Helper T-cells get involved.
CK-12 Foundation
Ck 12: Biology: Cell Mediated Immune Response
[Free Registration/Login may be required to access all resource tools.] Describes the roles of T cells in a cell-mediated immune response.
Wikimedia
Wikipedia: Helper T Cell
This encyclopedia article from Wikipedia on helper T cells discusses what Helper T cells are and the function of the CD4 protein.