In the maintenance of human health, systems have been designed in us to surveil and deal aggressively with invasion by pathogens such as bacteria, viruses, helminths etc.  Likewise, our bodies have been designed to detect and weed out rapid out-of-control cell growth because this generally signals the onset of cancer.  

Sometimes however, the surveillance mechanisms can be overly sensitive, leading to the diseases of autoimmunity.  Examples of such disease includes type 1 diabetes, rheumatoid arthritis, alopecia areata and lupus erythematosis.  Here, instead of protecting us against outside attack, the body, in a manner of speaking, attacks itself.  Microscopic photographs of autoimmune lesions contain white blood cells which have been described as a swarm of bees.