We head back into the passages after last week’s discrete section. However, we find that passage and discrete questions are not mutually exclusive. How?
Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.
[02:56] Passage 4 (Questions 18 – 22)
Eosinophils are multifunctional leukocytes implicated in the pathogenesis of numerous inflammatory processes, including tissue injury, infection, and allergic reaction. Eosinophils have been shown to possess the ability to perform multiple immune functions, including antigen presentation and exacerbation of inflammatory responses through their capacity to release a range of preformed cytokines and lipid mediators. For example, eosinophils can induce tissue damage and dysfunction by releasing an array of cytotoxic granule proteins, one of which is eosinophil cationic protein (ECP). This release is termed degranulation.
Note: Words we thought are important have been highlighted above. You don’t have to memorize the word. You’re highlighting them because you want your eye to be able to jump to the important part of the passage when you inevitably come back to it during questions.
[05:22] Paragraph 2
Human natural killer (NK) cells are large granular lymphocytes that can play a cytotoxic role against stressed, transformed, or infected cells. NK cells are known to have immunoregulatory effects on immune cells, such as T cells, B cells, monocytes, and neutrophils, through cell-cell contact and secretion of various soluble products. However, few studies have evaluated the potential interactions between NK cells and eosinophils. Researchers designed a series of studies with the goal of characterizing activating or inhibitory effects of NK cells on eosinophils in vitro.
Note: This passage has given us a lot of information as highlighted in bold above. And now we know they’re going to tell us about the studies. They told us what they were looking for. And now they’re going to tell us how they did it.
[07:30] Paragraph 3
First, eosinophils and NK cells were cultured in round-bottom microwell plates at different ratios of NK cells to eosinophils in the presence of interleukin-5 (20 ng/mL). Eosinophil activation was evaluated after 3 and 12 hours by the measurement of cell surface expression of the early activation marker CD69 (Figure 1).
Figure 1 Eosinophil activation as measured by percent of CD69-positive cells after 3 and 12 hours of co-culture (*p < 0.05, **p < 0.01, ***p < 0.001)
Note: Again, the keywords we think are important have been highlighted in bold above. Now, we see in the figure that the x-axis is always the different ratios and how much NK versus how much is eosinophil.
Notice that eosinophil is increasing and we also see the percentage of the CD69 expression in the eosinophils. And then we see all these long lines at the top, which is telling us the statistical significance.
Then there’s a little code at the bottom or a little key in the caption, saying that:
(*p < 0.05, **p < 0.01, ***p < 0.001)
So all of these lines are showing where there’s a statistical significance between the thing at the beginning and the end of the bar. And we see a statistical significance between the 1:1 and the 10:1. There’s statistical significance between the 0:1 and the 5:1, 0:1 and the 10:1.
The long bars on the top can be a little confusing sometimes to students. But you’re looking at the two groups that are at the ends of the bars, and that’s where there’s a significance between them.
[10:39] Paragraph 4
Next, researchers aimed to assess the effect of NK co-culture on eosinophil degranulation. After 3 and 12 hours of co-culture, samples were centrifuged at 1500 rpm, and ECP levels were measured in the supernatants (Figure 2). No ECP was detected in supernatant culture of NK cells alone.
Note: We’re highlighting “the effect of NK co-culture on eosinophil degranulation.” We were told what degranulation was in the first paragraph. So we know what we’re examining and that is what happens to eosinophil degranulation if we put NK cells in the co-culture.
We also highlight “centrifuged” which means we’re spinning it really, really fast so we can get all of the solid things down at the bottom and this liquid or the supernatant above that. And we’re actually examining the supernatant and then we highlight “ECP levels were measured in the supernatant” so we remember which part of the centrifuge results we should care about.
Lastly, we see that there was “no ECP detected in NK cells alone.” From my own eye, I think I’ll just highlight no ECP detected, and then NK cells alone, because otherwise, that whole sentence is yellow. And that kind of throws off my visual acuity.
So from here, we can see that no ECP was detected in the NK, only supernatants – but in everything else, we got varying levels.
[12:12] Figure 2
Figure 2 Eosinophil cationic protein (ECP) levels in eosinophil supernatants after 3 and 12 hours of co-culture (*p < 0.05, **p < 0.01, ***p < 0.001)
Note: In Figure 2, we see the same thing. The ratio is on the x-axis. There are the 3-hour and the 12-hour. But on the y-axis, instead of the percentage of the CD69, we’re seeing the ECP concentration in these amounts. So you can see the ECP level in your supernatants.
Looking at Figure 2, we can then see that the levels of ECP are increasing as the ratio increases. And so the implication here is that having that NK presence is going to promote that degranulation. So I can see that is increasing, it’s pretty spread on those 10:1 ratios, but we also see a statistical significance between the 10:1 and the 0:1. So that helps us know that there is some increase, and it appears to be going in a linear fashion.
[13:26] Question 18
Which of the statements below is supported by the experimental results, as shown in Figures 1 and 2?
- The duration of Eos co-culture with NK cells directly and linearly correlates to the amount of ECP found in the supernatant after centrifugation.
- Cells cultured with a 1:1 NK-to-Eos ratio displayed statistically similar levels of activation to cells cultured with a 5:1 NK-to-Eos ratio, as measured by CD69 expression.
- NK co-culture stimulates Eos activation while inhibiting degranulation.
- Co-culture with NK cells significantly increased Eos degranulation in all groups, as compared to Eos cells cultured alone.
A – We see that the Y axes are actually really different in scale. So for the 3-hour, we’re going 0-25-50 and going up in increments of 25. But in the 12-hour one, it’s going up in increments of 250. So that increase looks real different. It looks very similar and this is such a common trick, where they give you the Y-axes messed around with so that the graphs look really similar. But it doesn’t actually look like that because these scales are so off. So that makes us very suspicious that it is linear.
B – So now we’re talking about Figure 1, which is where the CD69 expression is. So 1:1 versus 5:1 and they don’t give us time here. So let’s just look at one of the graphs for now and let’s try to stick with the 3-hour. And 1:1 displayed statistically similar levels of activation to cells cultured with 5:1. And we can see that the statistical significance bars at the top none of them cover 1:1 and 5:1. So they’re not statistically significantly different. And so we can potentially say they’re similar.
C- We were measuring NK co-culture on eosinophil degranulation. So it wouldn’t inhibit degranulation so this answer choice doesn’t seem to make sense.
D – We know that’s not true because usually there’s a range of at least one extra jump to get statistical significance.
Correct Answer: B
Just an important note for answer choice B where it’s like a double-negative version saying “which one of these does not have statistical significance between them?”'A lot of times people remember the phrase statistically significant, but they don't remember if it's difference or similarity. So that's something to remember.'Click To Tweet
There’s a statistically significant difference when you see the asterisks or the little cross. And that’s important because it’s an easy trap.
[18:17] Question 19
Assuming that ECP was named for its electrical charge at physiological pH, which of the following must be true?
- The primary structure of ECP contains more acidic residues than uncharged residues.
- The primary structure of ECP contains more basic residues than uncharged residues.
- The primary structure of ECP contains more acidic residues than basic residues.
- The primary structure of ECP contains more basic residues than acidic residues.
The uncharged residues are uncharged so they’re not going to contribute either way. So whether or not they are more acidic than uncharged is irrelevant. The number of uncharged doesn’t matter because they’re not going to take a positive charge. So we can cross out A and B.
Looking at C and D, the differences would become between the “more acidic than basic” or “more basic than acidic.” And so this actually requires us to go outside the passage and think about our knowledge of amino acids.
The acidic ones are the ones that adopt a negative charge. And so that would lead to an anionic protein. The basic ones are the ones that become a positive charge and acidic ones become negatively charged. You need to memorize this.
And so just for that reason, more basic residues than acidic residues means more positive charges than negative charges and so it means cation. Remember that cation is a positive charge.
There is actually very little of the passage here so this is a pseudo-discrete question. One word was from the passage that “cationic protein” was the only word we needed to know that it was a positive charge. So the correct answer here is D because it’s more basic and more basic means more positive charges.
Correct Answer: D
[23:39] Question 20
The researchers chose to co-culture all samples in the presence of interleukin-5, a cytokine. What is the most likely reason for this decision?
- Interleukin-5 facilitates degranulation in NK cells.
- Interleukin-5 inhibits the cytotoxic effects that NK cells have against eosinophils.
- Eosinophils die rapidly when not exposed to interleukin-5.
- The researchers were directly testing the effect of interleukin-5 on eosinophil activity.
A – It’s the eosinophils that are being degranulated so this answer choice doesn’t make sense because they’re talking about the wrong cell.
B – This doesn’t seem right because the whole setup of this study is to show the cytotoxic effects even though it’s in interleukin-5. Also, the cytotoxic effects of the natural killer cells are specifically against stressed, transformed, or infected cells, not healthy ones. So this would introduce a whole bunch of problems if that was the point of introducing interleukin-5 . So that seems just fully not part of the experiment.
D – It just says they were in interleukin-5 so we can cross this out.
C – This is very much a “process of elimination” question. But then if we think about it a little bit, maybe it’s just the thing they needed for any of this to happen at all. They just needed to be exposed to it in order for any of the effects that we saw later to happen. They didn’t change it, they didn’t modify it. They just had to be exposed to it.
Correct Answer: C
[27:01] Question 21
Eosinophils play a major role in the promotion of the inflammatory response, which is marked by increased vasodilation, blood vessel permeability, and pain. The inflammatory response is classified as:
- a function of the adaptive immune system.
- a function of the innate immune system.
- a function of passive immunity.
- a function of antibody-mediated immunity.
This is another pseudo-discrete question. So eosinophils are a major part of the inflammatory response as found in the passage.
A – It’s like we get a vaccine, and we learn what needs to fight, adapting to new situations. That doesn’t seem like what eosinophils do. So we can cross this out.
D – This is similar to answer choice A as well so we can cross this out too.
Now, we’re left between B and C. So passive vs. innate immunity.
C – Passive immunity is when you receive antibodies from an individual. It’s what you get when you’re a baby from breastfeeding. It’s not your antibodies and they’re not fully long-lasting. That’s why babies are so prone to getting sick because they haven’t built up their immune systems yet. So passive is “passed on.”
B – Innate immune system is your natural-born, without having to learn anything. It’s what’s pre-existing, it’s already there. The inflammatory response is very nonspecific. It’s just your body freaking out but with no direction. Your antibody-mediated and adaptive immune responses are specific. It’s like “I met this pathogen before. I learned it. I built antibodies. I can fight it.” So innate is our nonspecific group of responses.
Correct Answer: B
[30:26] Question 22
A typical eosinophil would be expected to differ from an erythrocyte in that the eosinophil:
- would not contain a membrane-bound nucleus.
- would contain a membrane-bound nucleus.
- would not be expected to be found in human blood.
- would be expected to be found in human blood.
Answer choices C and D are both wrong because the eosinophils are found in blood, and so are erythrocytes. So we can cross both of those out.
Think about the shape of the red blood cell or erythrocyte. It looks like a doughnut and in the middle, it seems like it’s making extra space for the hemoglobin molecule because they don’t have a nucleus. They don’t need a nucleus. They just need to carry oxygen.
Hence, eosinophil would contain a membrane-bound nucleus.
Correct Answer: B
[32:13] Final Thoughts
That was a very discrete passage section. You will find some comfort in knowing that there are plenty of pseudo-discrete questions in passages. A lot of it comes down to learning a lot of science and high-minded scientific principles when we’re studying for the MCAT.'When doing passages and reviews, discrete and passage are not mutually exclusive categories.'Click To Tweet
And then, in reality, remember dumb little mnemonics. For instance, the red blood cell looks like it has a hole in it because someone has stolen the nucleus. Some of this quick recall is to do it in the least taxing way possible. Otherwise, if you have to think through everything and you have no mnemonics to grab onto, it’s just tiring. And so, incorporating mnemonics is easier for you to go through questions like those because you’re not thinking so hard.
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