We’ve come full circle, I hate the immune system...
Okay but what are hypersensitivity reactions?
They are abnormal and/or exaggerated immune responses to substances that are inhaled, ingested or that come in contact with tissues.
There are four types of hypersensitivity reaction and they involve different parts of the immune system. They are also rather pervasive and are either directly involved in the pathophysiology of a disease or act as underlying complicating factors.
Type I Hypersensitivity: Involve IgE antibodies activating Mast Cells and causing massive release of pro-inflammatory cytokines
Type II Hypersensitivity: Involve IgG and/or IgM antibodies bound to cell-surface/matrix antigens which also stimulate inflammation
Type III Hypersensitivity: Involve antigen:antibody complexes building up and damaging tissues both physically and as pro-inflammatory complexes
Type IV Hypersensitivity: This is the only cell-mediated (rather than antibody mediated) reaction and it is caused by overactive T cells, it is also known as Delayed-Type Hypersensitivity
The take home message is that the immune system is built for killing/neutralising threats (with some regeneration afterward) so any deranged responses are highly toxic to tissues and is the main reason why my favourite sentence, “pathology is immune mediated” is so ubiquitous in medicine.