Normally, the body's immune system responds to the invasion of pathogen. In the case of HIV, for example, an infected host cell alerts the immune system by secreting interleukine-2 (IL-2), a special protein (called a cytokine) that acts as a key messenger for many cells of the immune system.

IL-2 acts as a T cell growth factor, promotes NK proliferation and stimulates B cell growth (cells that produce antibodies). Together, these "soldier cells" attack foreign pathogens like viruses, and help to destroy them. From the first encounter with the invader, the immune system keeps a memory of what happened and specialized "memory" T and B cells are established as guardians in the host's body. The next time the invaders try to enter, they will be swiftly attacked and disarmed. .