Hi everyone, and welcome to Biology Professor. Today, we're talking about the four different
kinds of hypersensitivities. So first, what is a hypersensitivity? Well, a hypersensitivity is
an undesirable or an actually harmful reaction of a person's immune system.
Sometimes also called an intolerance, specifically when we're talking about sort of more undesirable
things rather than harmful things, but an intolerance can be used to discuss all of them.
So now let's talk about those four different types starting with type 1. Type 1 hypersensitivities
are when a specific antibody, specifically IgE, recognizes soluble antigen. Remember here that Ig
stands for immunoglobulin which is another name for antibody, IgE is one of the major
types of antibodies. So IgE recognizes soluble antigen. What does this mean? An antigen is just
the thing being recognized by the immune system. In this case, it is something soluble, so it's
not something that is cellular or at the tissue level, it's recognizing just something that's kind
of dissolved and floating around in the body. In particular with type 1 hypersensitivity,
IgE is recognizing things like pet dander, mold, pollen... things like that that we think of as
being related to seasonal allergies. And so IgE binds to that soluble antigen when it finds it.
This triggers an immune system cell called mast cells to do something called to degranulate.
So the mast cells degranulate. And all that means is they basically just spill out their
guts. These mast cells are full of signaling compounds that are then just released into
the body. And the major one here that we're concerned with is something called histamine.
You might recognize that word because it is antihistamines that we take when we want to
relieve allergy symptoms. And that's because histamine release causes the allergy symptoms
that we associate with a lot of these seasonal allergies or pet allergies. So I'm talking about
coughing, sneezing, red eyes, watery eyes, hives... those are all being caused by this
massive histamine release in the body and antihistamines can help to control those
symptoms. So that is type 1 hypersensitivity. Now let's talk about type 2. Type 2 is also
mediated by antibodies but by different kinds of antibodies. Rather than IgE, the antibodies that
are in these type 2 hypersensitivities are IgG and IgM. And they recognize,
rather than the soluble antigen, stuff just happening to be floating through the body,
they're recognizing cellular antigens. So antigens that are actually on the surface
of the host cells. And what this does is when these antibodies recognize and bind to cellular
antigens they're essentially binding to cells and causing a lot of damage to those cells. This can
be termed "cyto" (cyto for cell) "toxicity". So cytotoxicity is when cells are being damaged.
Some examples of type 2 hypersensitivity are the recognition of certain self-antigens.
Self-antigens means something that is self, something that belongs to the host, that is not
a foreign entity and this results in conditions known as autoimmune disorders or autoimmunity.
So some of these type 2 hypersensitivities, where you have IgG and IgM, recognizing these cellular
antigens and causing damage to the cells, this can be sort of self cells, and it can also in
particular be red blood cells. So the ABO groups, if you've heard like Type A, Type B, Type O,
Type AB, that has to do with carbohydrates that are on the surface of red blood cells. And also,
if you've heard about being like A-positive or O-negative, that positive and that negative is
referring to Rh antigens, which are also just PROTEINS (not carbohydrates) that are stuck on to
red blood cells. And so what can actually happen here is if the immune system, if these antibodies,
recognize either the ABO or the Rh antigens on red blood cells as being foreign, they start to
attack red blood cells and this is what can be dangerous with blood transfusions. So hemolytic
blood transfusions when someone gets a transfusion of blood that is not sort of compatible with their
blood type, that is because these antibodies are attacking these red blood cells that are being
put into the body from a foreign source that aren't compatible. Also you may have heard of
hemolytic disease of the newborn. This is when a mother's immune system actually starts to attack
the baby's blood when the baby is sort of still in in utero or just afterwards because of this
sort of incompatibility where the mom's antibodies start to attack, attack on recognizing the baby's
different antigens. Now let's talk about type 3 hypersensitivities. Type 3 hypersensitivity are
specifically characterized by these things called immune complexes. Immune complexes are when you
have aggregations of antibody and soluble antigen so not just one antibody recognizing one soluble
antigen, but multiple antibodies recognizing more than one soluble antigen at a time,
binding to more than one at a time. And then those same soluble antigens will be bound by
other antibodies and you just get this large mass of all of these soluble antigens that are being
recognized by all of these antibodies at once and that's called an immune complex. And the immune
complex will deposit into blood vessel walls, and once it's deposited into the blood vessel walls,
it causes a couple of things that happen. First of all, it causes complement cascades. Complement
are types of proteins that are part of the immune system that cause this cascade where
all these complement proteins are activating other complement proteins and they eventually
lead to something called a membrane attack complex where they form a hole in a cell and bust it open
and kill it. And when this is happening, this is happening in the body to host cells rather than to
pathogens when complement cascade results in the death of pathogen so it's a good thing, when they
result in the death of host cells like in type 3 hypersensitivity, that's a bad thing. Something
else that happens when these immune complexes deposit into blood vessel walls is neutrophil
degranulation. We had previously mentioned in type 1 hypersensitivities with mast cells
degranulating, that means they just spill forth and sort of secrete all the components that are
held inside their cells. And for neutrophils this means a lot of sort of cytotoxic enzymes, a lot of
enzymes that are meant to degrade and break down things, and so when complement starts to have its
cascade and neutrophils degranulate, there's all of this sort of damage that's being done, and this
results in widespread inflammation in the body. So lots of inflammation, lots of tissue damage,
an example of this is the autoimmune disorder lupus, which you might have heard of before and
lupus is where antibodies bind either to self DNA or to self proteins. Remember when we have
self antigens being recognized antibodies, we call that autoimmunity. So this is another place where
autoimmune disorders happen, specifically being caused by these immune complexes, and causing the
sort of systemic diseases like lupus. Now let's move on to the final type of hypersensitivity,
type 4. Type 4 is different than the first three types in particular because it is cell-mediated.
It is actually regulated by T cells. So whereas types 1 and 2 and 3 all involve antibodies doing
something that was going to harm the host, type 4 it's about T cells doing something wrong. So
the T cells can be Th1 or Th2 cells, different kinds of T helper cells, it can also be cytotoxic
lymphocytes, which is a different kind of T cell. But either way, it's considered cell-mediated
rather than antibody-mediated and it's also known as a delayed type of hypersensitivity,
so delayed hypersensitivity. And all this means is that the T cells, they start to attack something
that otherwise shouldn't be harmful in order to get it out of the body. This results in things
like latex allergies, poison ivy, the rash, the dermatitis that you get from contact with poison
ivy, and also transplant rejection. So if you hear about someone who maybe got a lung transplant and
then their body rejected it, that is because of a type 4 hypersensitivity where the T cells
recognized either the latex or the poison ivy or the the lung transplant, one of these things,
and said "hey, that shouldn't be here" and they have this really strong overreaction to eliminate
what they view as a threat, and it results in either sort of an allergy or a rash or at worst
a major rejection of the needed organ transplant. And known as a delayed hypersensitivity because it
can take several days sometimes for these things, for the symptoms, to appear. So several days
following exposure in some cases. And so that is sort of it for the four types of hypersensitivity.
I have another video specifically on on antibodies, on antibody structure and function,
and so if you're interested in learning more about some of these antibodies, what they look like,
what they do, then please check out that video and thanks for watching Biology Professor! :)