Question: When and why does the immune system malfunction?

Carmen Denman answered on 16 Mar 2015:

The immune system may malfunction when it encounters a pathogen that is an expert at beating it. The immune system may also malfunction when it turns on itself – either by producing allergic reactions to things like grass seed, or pollen, or by the onset of an auto-immune disorder. Why these things start in the first place is a complicated question – and for every person and every auto-immune disorder there could be a different reason. No simple way to answer this question! Here is just a short list of some more common autoimmune disorders – each with specific systems and causes:

Addison’s disease
Celiac disease – sprue (gluten-sensitive enteropathy)
Dermatomyositis
Graves’ disease
Hashimoto’s thyroiditis
Multiple sclerosis
Myasthenia gravis
Pernicious anemia
Reactive arthritis
Rheumatoid arthritis
Sjogren syndrome
Systemic lupus erythematosus
Type I diabetes