T cell: a white blood cell called a lymphocyte produced by the thymus; central to the first line of immune defense called cellular immunity; includes two groups of cells: CD4s and CD8s.

CD4, or helper, cell: a T cell that stimulates other immune cells called macrophages to kill infected cells; also sends signals to B cells to produce antibodies to specific microbes (see photo above [image not available]).

CD8, or killer, cell: a T cell that directly kills infected cells by releasing chemicals that destroys them.

Memory cell: a T cell that was once activated with antibodies to fight a specific infection and now carries proteins on its surface to recognize the invader if it appears again.

B cell: a white blood cell called a lymphocyte that is produced in the bone marrow; each B cell makes antibodies that attack specific microbes.