Immune system animations

The immune system is a vast collection of tissues and cells that work together to keep the body free of infection. In doing so, the system has to balance between hypoactivity, which allows infection to reign, and hyperactivity, which causes allergies and autoimmune conditions. A complex system of local and long-distance signals allow for the system to respond appropriately. Long distance signals place the body and the immune system on alert. Local signals tell arriving immune cells that they are entering the war zone, and to draw their weapons (keeping the weapons holstered until they arrive at the battle field reduces friendly-fire incidents)

There is also the problem of recognition - immune cells are worthless if they don't know what to attack, since they ignore pathogens (bacteria, etc) and attack host cells (self). The system by which immune cells are schooled requires specialized environments and signaling processes.

This site shows animations that explain many of the intricacies of the immune process. It's much more than you need for the MCAT, but very useful for those of you already into med school.