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This lesson will cover type IV hypersensitivity. We’ll discuss the basics of how and why it occurs, the cells involved, and the most common types of diseases and conditions that are associated with this hypersensitivity reaction.

Slower Processing Speed

If you’re someone who is constantly on the edge of new advances in computers and electronics, then you are undoubtedly concerned about many factors, one of which is the processor speed of your computer.

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Your immune system has a couple of different speeds at which it can process and react to certain foreign particles, or antigens. It can be like the newest and coolest Mac or like a really slow and cranky PC. No offense is meant to any PC owners out there, by the way!

Type IV Hypersensitivity

Our fast and spiffy Mac is like a type of hypersensitivity called type I, or immediate, hypersensitivity. That’s because it occurs within seconds or minutes after exposure to some kind of antigen.This is in contrast to our old and cranky PC, which is like our type IV hypersensitivity reaction, also called the delayed-type hypersensitivity reaction. That’s because instead of taking seconds or minutes to react to an antigen, it usually takes 24 to 48 hours to develop.

You can think of the antigen as a USB drive being plugged into our computer. With our PC it will take one or two days to open up the drive on our computer. That’s pretty slow.

T Lymphocytes

The other interesting thing about our old and cranky PC isn’t that it’s slow but that it also goes by another name: cell-mediated hypersensitivity.

This is because, unlike type I, II, and III hypersensitivity reactions, it doesn’t involve little proteins called antibodies. Instead, it involves cells called CD4+ T cells and CD8+ T cells, both of which are a type of white blood cell called a lymphocyte.What happens is, once our USB is plugged into the PC, or upon exposure to an antigen, the T cells are sensitized to the antigen. That is to say, the computer ‘reads’ the USB drive.

Upon re-exposure to the antigen, that is to say when the USB drive is plugged in a second time, these lymphocytes do one of two things. CD4+ cells secrete molecules called cytokines, which will promote an inflammatory response by attracting other inflammatory cells and molecules, whereas CD8+ cells, called cytotoxic T-cells, will directly kill any cell that harbors the antigen it is sensitized to.This response is actually a normal response of cell-mediated immunity to an antigen. However, when our body overreacts and is overly sensitized to the antigen, it can cause a very serious inflammatory reaction that ends up damaging our body in the process.

Disease Examples

Some famous reactions and diseases that are attributed to type IV hypersensitivity include:

  • The skin reaction test for tuberculosis
  • Type 1 insulin-dependent diabetes mellitus
  • Contact dermatitis due to poison ivy or other substances
  • Multiple sclerosis
  • Viral hepatitis
  • Crohn’s disease
  • Chronic transplant rejection
  • Graft versus host disease

As a more specific explanation, multiple sclerosis is a disease that involves the inflammation of and damage to the insulating layer of nerve cells in the central nervous system. In this disease, T cells seem to play a critical role. Something – we’re not sure what – triggers T cells to become sensitized to this insulating layer, called myelin.Once sensitized, the T cells enter the central nervous system (the brain and spinal cord) and begin to destroy the surrounding sheath around nerve cells using the processes I described before.

The destruction of this myelin causes the electric impulses traveling along the nerves to slow down. This poor electrical conductivity causes all sorts of problems, ranging from mood disorders to difficulty walking.

Lesson Summary

Hopefully you never have to deal with multiple sclerosis, a disease that involves the inflammation of and damage to the insulating layer of nerve cells in the central nervous system, since there is no known cure.This disease is an example of type IV hypersensitivity reactions, also called delayed-type hypersensitivity, which is mediated by CD4+ T cells and CD8+ T cells, which are the two main lymphocytes involved in type IV hypersensitivity reactions.

Learning Outcome

When you have finished this lesson, you should be able to recognize that Type IV Hypersensitivity is slow (24 to 48 hours) and needs T cells (lymphphocytes).

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