VCE Biology Question Thread

[vox nihili]

Don't bacteria produce antigens on their own MHC I markers?

Also, so you are saying that viruses dont actually possess MHC markers, but when they infect the host cell, they present their antigens on that host cell's own MHC I markers?

Bacteria don't have immune systems, so they don't produced MHC molecules.

MHC molecules are produced by human cells, not other cells. What these molecules do is take an antigen from another cell and present it on the surface. IN other words, they hold the antigens up.

[cosine]

Omg new concept every post i make ..

So, viruses infect host cells and then put their antigens on the MHC Class I OR II or the host cell? Is it class I?

[vox nihili]

Omg new concept every post i make ..

So, viruses infect host cells and then put their antigens on the MHC Class I OR II or the host cell? Is it class I?

Viruses infect host cells, they get chewed up by the cell, and then the cell uses MHC class I molecules to pop viral antigens on the surface.

[cosine]

Viruses infect host cells, they get chewed up by the cell, and then the cell uses MHC class I molecules to pop viral antigens on the surface.

Alright and then these MHC Class I molecules can be recognised by specific receptors of B and T cells or?

And so what about bacterial cells, if they don't contain MHC I molecules then how do phagocytes engulf them and recognise them?

In other words, do phagocytes engulf bacterial cells directly or do they engulf cells infected by bacterias?

So how exactly are the immune cells able to distinguish between self and nonself? Everytime I conclude an answer someone says something is incorrect.. I thought that immune cells had differing receptors that would recognise non-self antigens on MHC Class I molecules of pathogens, but turns out neither viruses nor bacterial cells have MHC I markers.

[chemzy]

If you were allergic to something and it caused a rash on your skin, where would the allergen that binds to B cell receptors be located?

Thanks

[Biology24123]

Alright and then these MHC Class I molecules can be recognised by specific receptors of B and T cells or?

And so what about bacterial cells, if they don't contain MHC I molecules then how do phagocytes engulf them and recognise them?

In other words, do phagocytes engulf bacterial cells directly or do they engulf cells infected by bacterias?

So how exactly are the immune cells able to distinguish between self and nonself? Everytime I conclude an answer someone says something is incorrect.. I thought that immune cells had differing receptors that would recognise non-self antigens on MHC Class I molecules of pathogens, but turns out neither viruses nor bacterial cells have MHC I markers.

No, MHC I are only recognised by cytotoxic T cells. Once the non self antigen is recognised by the cytotoxic T cell, the infected cell is killed.

Yes, APC's engulf bacteria directly

Proteins are constantly updated on the MHC I on cells. If these proteins are self proteins, then the cell is not affected and is left alone by CT8 (cytotoxic) cells. If the MHC I presents non self proteins (viral) then the cell is infected and will be killed by cytotoxic T cells

Remember that viruses are intracellular and most bacteria are extracellular. So it will be mainly viral proteins that presented on MHC I and not bacterial.

[cosine]

No, MHC I are only recognised by cytotoxic T cells. Once the non self antigen is recognised by the cytotoxic T cell, the infected cell is killed.

Yes, APC's engulf bacteria directly

Proteins are constantly updated on the MHC I on cells. If these proteins are self proteins, then the cell is not affected and is left alone by CT8 (cytotoxic) cells. If the MHC I presents non self proteins (viral) then the cell is infected and will be killed by cytotoxic T cells

Remember that viruses are intracellular and most bacteria are extracellular. So it will be mainly viral proteins that presented on MHC I and not bacterial.

Okay, that sort of clears things up for me. But I still do not understand how, how, how and how, how do phagocytes detect bacterial cells to engulf. Before we said receptors on immune cells are able to distinguish MHC markers on various pathogens, but if you said there are no MHC markers on bacterial cells, then how do phagocytes know to engulf bacterias?

[Biology24123]

Okay, that sort of clears things up for me. But I still do not understand how, how, how and how, how do phagocytes detect bacterial cells to engulf. Before we said receptors on immune cells are able to distinguish MHC markers on various pathogens, but if you said there are no MHC markers on bacterial cells, then how do phagocytes know to engulf bacterias?

Not sure who said the MHC markers on pathogens are recognised but that is wrong. I believe APC's recognise the cell walls of bacteria. Not sure how the body recognises good and bad bacteria

[cosine]

2006 Exam Biology, can someone please explain to me how and what they mean by this? How do immune cells specifically detect MHC markers

[vox nihili]

2006 Exam Biology, can someone please explain to me how and what they mean by this? How do immune cells specifically detect MHC markers

T-cells express receptors called T-cell receptors that can bind to MHC molecules. These TCRs are specific for an MHC molecule holding a particular antigen.

[cosine]

T-cells express receptors called T-cell receptors that can bind to MHC molecules. These TCRs are specific for an MHC molecule holding a particular antigen.

So are you saying that only T cells are able to distinguish between self and non-self?

[vox nihili]

So are you saying that only T cells are able to distinguish between self and non-self?

No, not at all.

As we've said before, B-cells are able to bind free antigens. This is how they recognise non-self. T-cells bind to antigens that are presented by MHC molecules. This is how they recognise non-self.

Remember that "self" molecules are just those molecules that are not recognised by the immune system at all.

[Shenz0r]

Okay, that sort of clears things up for me. But I still do not understand how, how, how and how, how do phagocytes detect bacterial cells to engulf. Before we said receptors on immune cells are able to distinguish MHC markers on various pathogens, but if you said there are no MHC markers on bacterial cells, then how do phagocytes know to engulf bacterias?

I think I mentioned this before. When you get inflammation, you get activation of the complement system. The complement system produces a compound called C3b which sticks to the pathogen. Phagocytes have receptors for C3b which allow the pathogen to be engulfed. Other times, antibodies will coat the pathogen and the phagocyte has receptors for these antibodies which help engulf it. Like this.



This is called "opsonisation" (and it's not needed in VCE)

Finally, some bacteria also trigger some specific receptors on phagocytes which initiates phagocytosis.

So are you saying that only T cells are able to distinguish between self and non-self?

No, other immune cells can also distinguish between self and non-self in other ways (ie through the PRRs on phagocytes, membrane-bound antibodies on B-cells). Only T-cells use MHC to distinguish between self and non-self.

[cosine]

I think I mentioned this before. When you get inflammation, you get activation of the complement system. The complement system produces a compound called C3b which sticks to the pathogen. Phagocytes have receptors for C3b which allow the pathogen to be engulfed. Other times, antibodies will coat the pathogen and the phagocyte has receptors for these antibodies which help engulf it. Like this.

(Image removed from quote.)

This is called "opsonisation" (and it's not needed in VCE)

Finally, some bacteria also trigger some specific receptors on phagocytes which initiates phagocytosis.

No, other immune cells can also distinguish between self and non-self in other ways (ie through the PRRs on phagocytes, membrane-bound antibodies on B-cells). Only T-cells use MHC to distinguish between self and non-self.

Thanks guys, so is there a reason why the examiners report stated that immune cells recognise with MHC markers to distinguish from self and nonself? I was in a bit of a predicament because I assumed they were not wrong, and I assumed that all immune cells actually interacted with MHC markers to recognise them or not..

So different immune cells identify self from nonself in different ways, right?

T-Cells: bind with MHC II molecules ( and also MHC I via viral infected host cells? )
B-Cells: transmembrane antibodies specific to each antigen.
Phagocytes: Some receptor mechanism that is not explored within VCE Bio?

Cheers guys, and I really appreciate your help all morning, must have asked the same question over and over again, but hopefully if the above is correct, then at least it worked, right? xD

Edit: If all nucleated cells possess MHC I markers with self-antigens presented on them, and if only T-cells interract with MHC molecules, then how do phagocytes bypass them?

[Shenz0r]

Thanks guys, so is there a reason why the examiners report stated that immune cells recognise with MHC markers to distinguish from self and nonself? I was in a bit of a predicament because I assumed they were not wrong, and I assumed that all immune cells actually interacted with MHC markers to recognise them or not..

So different immune cells identify self from nonself in different ways, right?

T-Cells: bind with MHC II molecules ( and also MHC I via viral infected host cells? )
B-Cells: transmembrane antibodies specific to each antigen.
Phagocytes: Some receptor mechanism that is not explored within VCE Bio?

Cheers guys, and I really appreciate your help all morning, must have asked the same question over and over again, but hopefully if the above is correct, then at least it worked, right? xD

Edit: If all nucleated cells possess MHC I markers with self-antigens presented on them, and if only T-cells interract with MHC molecules, then how do phagocytes bypass them?

The examiners report is really vague but yeah, only some immune cells use MHC markers to distinguish, and those are T-cells. What you just wrote about the different recognition mechanisms is correct.

As for the last question, well your phagocytes can still actually engulf your own cells if they're too stressed, if they're infected and if there's inflammation. It's just that for most healthy cells, they won't express molecules and factors that tell the phagocyte to chomp them up, whereas for bacteria, they will express foreign structures and they may get coated by antibodies etc.