T-cell activation

[zerofantasy]

My question is on immunology. Not sure if it belongs here but anyway...

I am a little confused on T-cell activation.
I know that:
1. In the secondary lymphoid tissue, we have naive, mature T-cells
2. Ag is presented to them by PAPC
3. T-cells differentiate and proliferate

Question 1: do these naive, mature T-cells already have CD8 or CD4 on the surface? I am assuming they do. If yes, what the heck do they mean by "differentiate"? Is it just that they become functional helper cells or killer cells based on the molecule they express?

Question 2: Do you get both, killer and helper T-cells each time an Ag is presented or do you get one type or the other based on the Ag?

Sorry if these are dumb questions. We've only covered chapter 1 and it lacks details and that makes me cringe. I tried looking at later chapters but they have too many details and confused me even more. I will only understand that stuff once I go through all the chapters before it! =)

[mith]

Try the wikipedia article on t-cells.

Killer's and helper's have complementary roles, if you have a virus in your system, it makes sense to go on a full out assault.

[biohazard]

1) T cells gain their cd4/cd8 type in the thymus before they enter the blood stream the first time. From blood they mostly head to secondary lymphod tissues in hope of finding their specific antigen there, presented by APCs. I assume here the differentiation means they become activated T cells (as opposed to naive T cells) and gain a "memory" T cell phenotype, which can be further divided into e.g. central memory or effector memory T cells (cd4 and cd8 cells), or Th1 and Th2 T cells (if they are cd4 cells), depending on the type of activation signals they receive. There are other categories as well, and it is likely we do not know nearly all of them yet.

2) Related to the 1st question: the body already has its NK and helper T cells, they just activate upon antigen stimulation. And naive T cells do not become NK cells or the other way round. NK cells are kind of borderline cases between adaptive and innate immune system: for example, some of them do not require the help of APCs but instead they can directly recognize certain molecules that lead to their activation - pretty much like the cells of the innate immune system. NK cells also have sub-categories among them if I recall correctly.

Hope this helps!