A 3 year old male is rushed to the emergency department
after consuming a peanut butter sandwich at daycare.
The patient is in acute distress.
His blood pressure is 60/40 and
heart rate is 110 beats per minute.
There is an audible inspiratory stridor and the
respiratory rate is 27 breaths per minute.
Upon examination of the patient's chest, we
find that it is covered in a maculopapular rash.
Intubation is attempted and fails
due to extensive laryngeal edema.
The decision for
cricothyroidotomy is made.
Which of the following is a most likely
the mechanism of this pathology?
Answer choice (A) - c5a production
Answer choice (B) -
release of interleukin-4
Answer choice (C) - deposition
of antigen-antibody complexes
Answer choice (D) -
and answer choice (E)
- c3b interaction
Now take a moment to determine the answer choice
for yourself before we go through it together.
Okay, let's work through
this question together.
The first thing to do is determine
what are the question characteristics?
Well this is an immunology question.
We have a patient who has consumed some type of
allergen and is producing a systemic response.
And the answer choices
all concern immunology.
And this is a 2-step question.
First thing we need to do is determine the
diagnosis of what's going on with the child.
And second, the actual underlying
mechanism of the pathology.
And of course, the stem is required in this case because again
it's a very complex question with lots of critical information.
So let's walk through this
Now step 1, we need to determine
the child's diagnosis.
Now the history that is provided in the clinical vignette supports
the diagnosis of the patient having a severe allergy to peanuts.
Now the rapid onset of hypotension, respiratory
distress and laryngeal edema and rash
is suggestive of what's called
ANAPHYLAXIS or an anaphylatic reaction.
Now this is what's called
a type 1 hypersensitivity.
So the patient has a type 1
hypersensitivity reaction to peanuts.
Now, now that we know that, we need to determine
the actual underlying mechanism of this process.
So type 1 hypersensitivity reactions arise due to the recognition
of peanut allergens as being considered a foreign antigen.
Now let's refer to our image, which is an image that
describes the four types of hypersensitivity reactions.
Type 1 being on the upper left,
Type 2 being on the upper right,
Type 3 being on the lower left and
Type 4 being on the lower right
So let's just start with Type 1 hypersensitivity which
is the answer to what the diagnosis is with the child.
Now as we can see by looking at
the image, we have an antigen
which is going to be the allergen in this case
which is going to be that little green dot.
Imagine it's part of a peanut and that peanut
component is a foreign antigen to this child.
And what we see is that the peanut antigen is
actually binding to the immunoglobulin E or IgE
and then that's actually activating the
Fc receptor at the immunoglobulin E.
And what happens is we have these type of
receptors on both mast cells and basophils.
So when it's happening, is we have peanut
allergen bind to Ige on mast cells or basophils,
we get a very classic reaction which is
degranulation of the mast cells and basophils
and release of vasoactive inflammatory
mediators such as histamine.
Now, interleukin-4, very high-yield, plays a critical role
in allergic disorders including food hypersensitivity
and is released.. and is involved
actually in antibody production.
So in this case, this is the
answer to our clinical vignette,
as the release of interleukin-4 is occuring in a type
1 hypersensitivity reaction to the food allergy.
Now, a very important to know and extremely
high-yield is the mechanism of type1 hypersensitivty.
This is something you
just have to memorize.
Now, the first exposure to an antigen does NOT provoke
a reaction but it does cause what is called 'sensitization'.
So in this case, the first time the child may have a
peanut sandwich, he did not get this gross reaction,
he just had sensitization to the process.
Now, during sensitization, IgE antibodies specific to the antigen
are produced and bind to the IgE receptors on mast cells.
Now that happens during the
Upon second exposure, which may have
been what happened here to this child,
the antigen binds to the IgE
antibodies that are now produced
and this causes massive degranulation of both
mast cells and basophils which release histamine.
And interleukin-4 plays a key role
in the initial antibody production
which is involved in the process of differentiating the B-cells
into plasma cells to actually produce the IgE antibodies.
Now let's go back to our image.
We thoroughly went through
the Type 1 hypersensitivity.
Now let's go to some high-yield facts to discuss the
Type 2, Type 3 and Type 4 hypersensitivity reactions.
So, Type 2 hypersensitivity is 'cytotoxic hypersensitivity'
which is generally mediated by IgG or IgM
which is going to react to surface antigens
and activate the complement system.
A typical example here which is extremely high-yield
is when you have ABO blood incompatibility.
So you have a person, say who's A
positive and they're given type B blood,
they will have a Type 2 hypersensitivty reaction
which will activate the complement system.
Extremely high-yield, take the
example of the blood incompatibility
and understand that you have complement
activation in Type 2 hypersensitivity.
Looking at our image,
we definitely see that.
We see the actual surface antigen, a cytotoxic
cell coming through with an Fc receptor
and it actually will activate the complement system
to produce a cytotoxic hypersensitivity reaction.
Now let's look at Type 3 hypersensitivity.
What occurs here is actually deposition of
circulating immune complexes in tissue.
A very classic and high-yield example
here is in rheumatoid arthritis.
Now if you look at the image, we have
immune complexes floating around.
And neutrophils in this case will again cause
complement activation along with the immune complexes
to cause the Type 3
So both Type 2 and Type 3 have complement
activation - extremely high-yield.
Now looking at Type 4 hypersensitivity, this
is a unique one, very important to recall.
This is a delayed recation, it can
actually take several days to occur.
And this is the only hypersensitivity
reaction that is NOT, and I repeat NOT
mediated by antibodies but
instead is mediated by T-cells.
So if you look at our Type 4 hypersensitivity, we see
an antigen coming in to our sensitized T-helper1 cell
which will then release cytokines and
cause the activation of macrophages.