T-lymphocytes...question..on well, everything

[SaintJude]

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Could someone maybe go through the ...well, entire information we need to know about T-cells on the MCAT?

It would be such great practice and a gift to SDN generations to come! 😛

 

[MedPR]

Could someone maybe go through the ...well, entire information we need to know about T-cells on the MCAT?

It would be such great practice and a gift to SDN generations to come! 😛

I have a bit more background knowledge than necessary for the MCAT, so I'll try to present only what I remember seeing in EK. I'm assuming they have it all covered.


CD4s are T-helpers. They secrete cytokines that induce actions in the other leukocytes.

CD8s are T-killers (cytotoxic). They kill infected cells.

T-cells cannot recognize free antigen (antibodies can) and must have the antigens presented by antigen presenting cells (dendritic cells, macrophages, b-cells, etc).

T-cells are made in bone marrow (red/hematopoietic) and undergo maturation (positive & negative selection) in the thymus.

 

[SaintJude]

Tried to contrast b/w T and B-cells. Am I missing anything?

 

[Temperature101]

Tried to contrast b/w T and B-cells. Am I missing anything?

That is a nice contrast...

 

[pfaction]

I would include MHC-I and MHC-II in there, our TPR book distinguished it across a page.

 

[SaintJude]

Tell me more..nothing on it in Kaplan.

 

[MedPR]

The difference between MHC-I and MHC-II are beyond the MCAT, but if you want to know...

MHC-I is on every nucleated cell in the body, including the antigen presenting cells. MHC-II is found only on antigen presenting cells (dendritic cells, macrophages, b-cells, etc).

After reading the above, your first question might be "So then all nucleated cells can present antigens?" And the answer is yes, MHC-I can present antigens just like MHC-II can, however MHC-I and MHC-II are recognized by different receptors. CD4 cells have MHC-II receptors and CD8 cells have MHC-I receptors. Hence, in a case of hepatitis (B & C, I think), CD8 destroys hepatocytes because the hepatocytes are presenting antigen on MHC-I.

So what you might need to actually know for the MCAT is that a professional APC will phagocytose an antigen, process that antigen, and then present it on MHC-II, which then gets recognized by CD4, which then secretes cytokines, which result in a cascade of events dependent on which cytokine(s) are secreted (b-cell proliferation, cd8 proliferation, etc).

 

[chiddler]

are you sure it's beyond the mcat?

i learned it in AP bio years ago :-x

 

[MedPR]

are you sure it's beyond the mcat?

I've taken the stance that any Bio not in EK is not necessary for the MCAT. Whether that's true or not I don't know.

However, I will say that I've taken this stance primarily because I have microbiology and a semester and a half of anatomy/physiology fresh in my brain so I'm hoping I'll be ok for Bio.

 

[ipodtouch]

Just a few questions:

1.) According to the table, T cells secrete Cytotoxic T cells.
Just wondering if this was correct.

2.) From what I remember:
-T memory Cells increase cytotoxic T cells
-B memory Cells create antibodies?

Also, wanting to confirm this.

 

[SaintJude]

Just a few questions:

1.) According to the table, T cells secrete Cytotoxic T cells.
Just wondering if this was correct.

2.) From what I remember:
-T memory Cells increase cytotoxic T cells
-B memory Cells create antibodies?

Also, wanting to confirm this.

1.) I was being a bit sloppy.
T cells mature and go on to become cytotoxic T cells following their activation after MHC-1 present antigen.

2.) T memory cells increase cytotoxic cells...yes we agree.
Yeah B memory cells remember the same pathogen for faster antibody production in the future

 

[411309]

There was actually a passage about the difference between MHC-1 and 2..I cant remember if it was tbr bio or tpr science workbook. They gave all the details in the passage though

 

[pfaction]

I've taken the stance that any Bio not in EK is not necessary for the MCAT. Whether that's true or not I don't know.

However, I will say that I've taken this stance primarily because I have microbiology and a semester and a half of anatomy/physiology fresh in my brain so I'm hoping I'll be ok for Bio.

Really, hm. TPR stated it across 2 pages, whoa.

 

[MedPR]

Really, hm. TPR stated it across 2 pages, whoa.

Well it doesn't hurt to have the background knowledge, but it probably won't be a discrete.

 

[Charles_Carmichael]

Tried to contrast b/w T and B-cells. Am I missing anything?

Not entirely sure what you meant by "secretes cytotoxic T-cells." That makes no sense. Think of it in simple terms:

• CD4+ T-cells are helper T-cells -- they secrete cytokines, which direct the immune response. If you think of the immune system as a football team, these cells are basically the quarterbacks of the immune system. They coordinate the offense against invading microorganisms, whether it's a cell-mediated response or a humoral response (you need these helper T-cells to fully activate B-cells for them to produce antibodies and differentiate).

• CD8+ T-cells are cytotoxic T-cells -- they act against infected cells and cancer cells.
• T-cells require peptides to presented on MHC molecules.
• B-cells recognize free antigen (no presentation needed).
• B-cells mature in the bone marrow.
• The receptor on the surface of a B-cell (which recognizes antigen) is a membrane-bound version of the soluble antibody it will produce.

Not sure what you mean by "Its T-memory cells increase cytotoxic T-cells" either. Both helper T-cells and cytotoxic T-cells can form memory cells.

I think specifics of the immune system and the immune response are beyond the scope of the MCAT (or the info will be given in a passage), so understand the basic concepts behind how these cells function without worrying too much about details.

 

[hellocubed]

After reading the above, your first question might be "So then all nucleated cells can present antigens?" And the answer is yes, MHC-I can present antigens just like MHC-II can, however MHC-I and MHC-II are recognized by different receptors. CD4 cells have MHC-II receptors and CD8 cells have MHC-I receptors. Hence, in a case of hepatitis (B & C, I think), CD8 destroys hepatocytes because the hepatocytes are presenting antigen on MHC-I.

So what you might need to actually know for the MCAT is that a professional APC will phagocytose an antigen, process that antigen, and then present it on MHC-II, which then gets recognized by CD4, which then secretes cytokines, which result in a cascade of events dependent on which cytokine(s) are secreted (b-cell proliferation, cd8 proliferation, etc).

Hey, wondering exactly what MHC-I and MHC-II receptors are.

If CD8 cells need MHC-I receptors in order to act on a cell, why does that cell even have MHC-I receptors? Wondering why a foreign invader would present its own antigens in MHC-I so that the host could destroy it.

 

[Charles_Carmichael]

Hey, wondering exactly what MHC-I and MHC-II receptors are.

If CD8 cells need MHC-I receptors in order to act on a cell, why does that cell even have MHC-I receptors? Wondering why a foreign invader would present its own antigens in MHC-I so that the host could destroy it.

MHC proteins display short peptides that the T-cell receptors look at. Class I MHC proteins display endogenous peptides (aka. fragments of proteins that are being made inside the cell) while class II MHC proteins display exogenous peptides (aka. fragments of protein that have been phagocytosed by a profession antigen-presenting cell, such as a dendritic cell, macrophage, or B-cell).

MHCs are required because T-cells cannot recognize "free antigen" like B-cells can. For the T-cell receptor to recognize an antigen, it has to be presented on an MHC. This is known as MHC restriction. If something is not being presented on an MHC molecule, a T-cell does not "see" it. Make sense? It's too dangerous to have T-cells recognize free antigen. They can cause a lot of damage if not appropriately activated.

Class I MHC molecules basically allow T-cells to survey what is going on inside of the cells in our body. Remember that humoral immunity is useless against infected cells (since antibodies cannot get inside cells). So the immune system needs a way to look inside cells; this is achieved via MHC I. Viruses and other pathogens that infect do not choose to be presented on MHC I. Rather, our cells have mechanisms that chop up cytoplasmic proteins and display them on MHC I. So, if a cell is infected by a virus and there's production of viral proteins in the cytoplasm, the protein-degrading machinery in our cells is not going to differentiate between what's our own cell's product vs. what's a virus's product. The protein gets chopped up regardless and gets displayed on MHC I. That way, the T-cells that do recognize the foreign antigen will be alerted to the fact that the cell is infected and can destroy the cell and stop the infection from spreading.

Make sense?