The immune system's basically the product of an arms race between humans and microbes. It's complicated because different mechanisms are independent: Adding a

The immune system's basically the product of an arms race between humans and microbes. It's complicated because different mechanisms are independent: Adding a new mechanism allows it to kill a few more microbes without subverting the effectiveness of existing mechanisms. Humans have an intrinsic advantage in that they're big creatures that can use strategies not available to single-celled organisms. E.g. having different specialist cell types that signal each other, or altering the homeostasis set point to change the ambient environment (fever).