Immune system

[chiddler]

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1. What activates T helper cells?

2. How are specific B cells activated? I know T helper cells activate them, but what I would like to know is how does T helper cell know that it needs to activate a specific B cell for the relevant antigen?

 

[typicalindian]

1. What activates T helper cells?

2. How are specific B cells activated? I know T helper cells activate them, but what I would like to know is how does T helper cell know that it needs to activate a specific B cell for the relevant antigen?

1. I believe T helper cells(CD4) are activated by recognizing MHC class 2 protein's that are located only on antigen processing cells (APC's) like macrophages or dendritic cells. The macrophage will take in the antigen via endocytosis, it will process the antigen and display peptide fragments from the antigen with MHC class 2 which is located on the APC's surface.

side note: T killer cells (CD8) are activated because they are able to recognize things that are associated with MHC class 1. Which is present on every nucleated cells in the body.

2. I don't think we need to know this level of detail for the MCAT because I believe the signal going from T cell to B cell is still being studied. Either way, this kind of information would most likely be given to us in a passage if we did need to know it.

 

[syoung]

For #2
It's not that it "knows". But rather B cells ingest antigens and present them to T cells who help activate them and turn them into plasma cells. B cells hard hard to turn into antibody secreting cells without helper T cells. On the same level, the helper T cells must also be activated or recognize the antigen through binding with receptor/antigen complexes on macrophages and other antigen present lymphocytes

EDIT: yar someone beat me to the punch. and this is supposed to be my major too...

 

[chiddler]

thanks very much for the helpful response.

 

[MedPR]

1. What activates T helper cells?

2. How are specific B cells activated? I know T helper cells activate them, but what I would like to know is how does T helper cell know that it needs to activate a specific B cell for the relevant antigen?

The others are correct about question 1, but I think there is a bit missing in regard to question 2. T cells don't really activate specific B cells in the sense that there are B cells off in some organ waiting for T cell activation. Naive B cells are in circulation and recognize antigens without help from T cells. When a B cell binds an antigen, it presents it to CD4 cells and the CD4 cells secrete cytokines, which cause the antigen bound B cells to mature and proliferate into different types of B cells like plasma cells or memory B cells.

So it's not that the T cell knows which antigen is there, it just stimulates the maturation, differentiation, and proliferation of the B cells that are in the area. Here's a good overview.


http://upload.wikimedia.org/wikipedia/commons/f/f7/B_cell_activation.png

 

[pm1]

A helpful mnemonic:

Help 4 you 2 (CD4 binds to MHC2 and activate T-Helper cells / well, CD4 is part of T-helper cells).