What does the immune system do?

The human immune system is a network of cells, tissues, and organs which combine to protect the body from internal and external threats. External threats are typically infectious agents, including bacteria, fungi, and viruses, while internal threats include cancer cells and sites of tissue injury. The immune system activates and regulates itself via extremely complicated molecular signaling between its constituent cells and the threats themselves. Understanding the myriad of pathways that regulate the immune system is critical to harnessing the body’s own defense system for therapeutic purposes.