We’ve been taking on the hypersensitivities one at a time. Today, we wrap up with number IV! Which it sort of makes sense that we do it last, not just because it is the last in order but also because it is a delayed type response, so it also is the last onset chronologically, taking at least 12 to 24 hours to get started.

Type IV is cell mediated unlike all the other ones. Both CD4 and CD8 T cells are involved, too. Basically, what happens is we get exposed to an antigen and the body responds. On the second round of exposure, Th1 cells over react and secrete to many cytokines. Things like IFN-gamma activate macrophages to start phagocytizing everything. All this overreaction cause granulamotosis, or a walling off of the area. Normally, that would be good but its not really serving an appropriate function here.

So, what are some examples? Obviously, there are quite a few, but the here are the high yield ones:

Allergic contact dermatitis
These are things like poison ivy, some medications or metal reactions.

Granulomas
This can happen wherever, just depending on how it came about.

Hashimoto’s thyroiditis
Autoimmune against the thyroid, so hypothyroidism comes about.

Multiple Sclerosis
Taking out that pesky myelin! Although, autoimmune disorder is not the only etiology for MS. Researchers are also considering genetics and environmental factors, so multifactorial is probably a more appropriate way to look at this one.

Rheumatoid arthritis
Immune system is probably attacking collagen here, causing arthrities, inflammation and general destruction.

It is also really important to note that Type IV serves as the reaction seen in a positive Mantoux test or Tuberculin reaction when testing for TB!