What happens when your immune system fails?

The immune system exists to protect the body. Most commonly, we think of the immune system as acting to prevent infection, or invasion of the body by foreign bacteria or viruses. However, the immune system also acts to protect the body from internal dangers - specifically, from somatic cells which have been damaged in some way and are now cancerous. 

Failure of the immune system, or immunodeficiency, results in a loss of this protective action. Frequent infection can be expected. Additionally, cancerous tumors will be more likely to form. Immunodeficiency can be genetic or acquired later in life.

We might also think of an overactive immune system as a 'failed' immune system. Autoimmune disease occurs when the immune system inappropriately targets and attacks the body itself (such as in haemolytic anemia) or when the immune system over reacts to environmental conditions (such as anaphylactic shock in response to an allergy).