This lesson will discuss something known as a type II hypersensitivity reaction. We will go over the principal mechanisms by which it occurs as well as the major antibodies involved. In addition, we’ll cover autoimmune hemolytic anemia and myasthenia gravis.
A Game of Tag
Tag, you’re it! I’m sure you have played tag at least once before. Your antibodies, little proteins floating in your blood, play a type of game similar to tag. They tag, or touch and attach, to certain things, such as bacteria, viruses, and so on. However, every now and then, this tagging turns deadly.
Imagine if you were playing tag with a friend and your light touch upon their body caused them to explode. That wouldn’t be a fun game to play!
Type II Hypersensitivity
Well, sometimes a similar explosion of the friendly cells in our bodies occurs in something known as a type II hypersensitivity reaction. This is sometimes called antibody-dependent cytotoxicity. That definition means that the toxicity, or destruction of our body’s cells, depends on antibodies. In this hypersensitivity reaction, imagine that you are a type of white blood cell called a plasma cell. Now, make a peace sign, or the Vulcan sign if you prefer Star Trek, with your fingers. The Y shape your fingers create sort of looks like an antibody that is made by plasma cells.
Normally, each antibody produced by a plasma cell is very specific to whatever it wants to destroy, such as a unique virus. It’s kind of like if you were playing tag and, instead of going after the nearest person you could touch, you specifically went after only your mortal enemy. Therefore, this type of game is great when it comes down to tagging viruses, bacteria, and other enemies of our body for destruction.
However, if your fingers kept tagging your friends (your own body’s cells) as opposed to your enemies (the pathogens), you’d be in trouble! Soon, you’d have no friends left to play with! This is the basic explanation of what occurs in a type II hypersensitivity reaction using antibodies called IgG and IgM.
Destruction by Connection
More specifically, the way this destruction occurs is a bit more complex. There are several ways by which your fingers will all of a sudden want to destroy your friends, or important cells in your body. One way this may occur is by indirect destruction. What I mean is if your enemy was holding hands with your friend and you tagged your enemy, it would automatically destroy your friend since she was connected to your enemy.
One example of this occurring in real life is when something like a drug molecule or a molecule of a pathogen sticks to the side of a cell, such as a red blood cell. In this case, the drug molecule is your enemy. The antibodies will stick, or tag, this enemy. However, the enemy is stuck to the side of the red blood cell, your friend.In some cases, the antibodies signal phagocytes, white blood cells that engulf and destroy pathogens, to come and destroy the frenemy combination. Everything and everyone is destroyed.
That’s because the phagocyte that performs this destruction cannot nitpick the drug molecule off of the red blood cell. It must take your enemy and friend down all at the same time.Other times, the same process I just described will lead not to phagocytosis, or ingestion by phagocytes, but to the lysis, or bursting open of those same red blood cells, through the formation of a little pore on the surface of the cell that has been tagged. In any case, this destruction of red blood cells is called hemolytic anemia, and when your own antibodies destroy your red blood cells, it is called autoimmune hemolytic anemia. In both the phagocytic and lytic outcomes, antibodies partner up with other protein molecules, called complements, to accomplish these tasks.
With that in mind, another way by which your two fingers, or antibodies, may be misdirected to destroy your friend instead of your enemy is a bit more sinister. Sometimes you, the plasma cell, will act like a complete drunk and will accidentally tag all of your friends, even when no enemies are around, because you aren’t thinking straight. Likewise, sometimes our body makes antibodies, called autoantibodies, that are directed against absolutely normal cells or receptors without any inciting cause, such as a drug molecule or a molecule of a pathogen sticking to that cell or receptor.In any case, these autoantibodies mark cells, like red blood cells, for destruction as per the methods discussed previously. These antibodies may also bind to receptors and cause serious dysfunction. One famous example of this is myasthenia gravis, which is an autoimmune disease that causes muscle weakness and fatigue.
In this disease, autoantibodies, that is to say antibodies directed against auto, or self, stick to certain receptors in a place where nerve cells meet muscle cells. The nerve cells release chemicals that bind to receptors on muscle cells and cause muscular contraction during movement. Well, autoantibodies tag and stick to these same receptors, blocking the chemicals released by nerves from exciting the muscle cells. This means muscular contraction cannot occur nearly as effectively as before.Other diseases involving type II hypersensitivity reactions include:
- Graves’ disease, which affects the thyroid
- Pernicious anemia due to decreased absorption of cobalamin, or vitamin B12
- Rheumatic fever, caused by the bacterium Streptococcus pyogenes
As you can tell, there are many diseases attributed to type II hypersensitivity reactions, sometimes called antibody-dependent cytotoxicity. In these reactions, antibodies known as IgG and IgM attack our own body’s cells and cause their phagocytosis or lysis.
Most notably, these antibodies attack red blood cells and cause autoimmune hemolytic anemia, which is a condition where your own antibodies destroy your own red blood cells. Sometimes these autoantibodies forego attacking cells and instead attack receptors to alter the function of your body as per a condition called myasthenia gravis, which is an autoimmune disease that causes muscle weakness and fatigue.
Once you’ve finished with this lesson, you will have the ability to:
- Describe what happens in a type II hypersensitivity reaction
- Explain what causes autoimmune hemolytic anemia and myasthenia gravis
- Identify other diseases involving type II hypersensitivity reactions