T cells go through a complicated process to mature before they can travel the body and patrol. It is during this process that they become Killer or Helper T cells.

Origin

T cells come from stem cells in the bone morrow and are sent to the thymus to mature.  The “T” in T cells is named after the thymus. The thymus is an organ behind your breastplate which helps the new T cells grow up.

The most important lesson the T cell learns is how to distinguish between healthy self and nonself. 

Your body only wants T cells that only attack intruder organisms and infected cells. You don't want them to attack healthy cells! So, any T cell that has a receptor shape that recognizes your healthy cells  is eliminated before it leaves the thymus. This is why our immune system doesn’t attack our own bodies. 

How do T cells Develop?

T cells identify cells by the pieces of proteins that cells present to them on their surfaces. The pieces of a germ are called antigens (read about antigens and identifying cells). These pieces are held by a protein called MHC. All cells use MHC to show T cells pieces of proteins.

When T cells are developing, they have to pass 2 tests. Only 2% pass! Here is what they have to do.

T cells have to be able to see the MHC on other cells. If they can't see the MHC, they'll never know if the cell is infected or healthy. T cells that don't see MHC are eliminated and never leave the thymus.

T cells must not bind to your own antigen shapes! If a T cell recognizes your own shapes, it is eliminated and never leaves the thymus. This makes sure that your T cells don't attack your own body.

But, sometimes T cells that recognize your own shapes sneak out. These T cells will attack your own body and cause autoimmunity.

Becoming Helpers or Killers

Some T cells become Helper T cells and some become Killer T cells.  Helper T cells are like the Police Chiefs of the immune system: they tell the other immune cells what to do and when to do it. Killer T cells can destroy infected cells, but need permission from a Helper T cell first.

T cells have a complicated task: they must be able to bind to (almost) all cells in the body, and they must be able to tell whether the cells are healthy or unhealthy. The maturation of the T cells is a complicated process that tests for these two abilities. This maturation process aldo determines whether each cell becomes a Helper or a Killer T cell.

Origin

T cells come from stem cells in the bone morrow and are sent to the thymus to mature.  The “T” in T cells is named after the thymus. The thymus is an organ behind your breastplate which helps the naïve T cells learn to develop into specific T cells.

T for Thymus

The most important lesson the T cell learns is how to distinguish between healthy cells (which carry self antigens) and unhealthy cells or germs (that carry nonself antigens). Basically, they learn to only attack intruder organisms, infected cells, and not healthy cells! Any T cell that binds to self antigens is eliminated before it leaves the thymus. Only T cells that mature into specific cells are allowed to leave the thymus. This is one of the main reasons why our very potent immune system doesn’t attack our own bodies. 

How do T cells Develop?

T cells identify cells by the pieces of proteins that cells present to them on their membranes; these pieces are called antigens (read more about antigens and identifying cells). Healthy body cells produce self antigens, and unhealthy body cells or germs produce nonself antigens. Most body cells have a special group of proteins - the Major Histocompatability Complex (MHC) whose job is to display these proteins. The MHC is a common group of proteins the T cells can grab onto to observe the antigen.

When T cells are developing, they go through the "gauntlet" that determines if they will survive. Only 2% make it! Here is what they have to do.

 T cells need to prove they can bind strongly enough to the MHC. Since MHC is the protein that holds the antigen, any T cell that can't bind MHC is useless. So, T cells in the thymus are exposed to MHC carrying a self antigen. Any T cell that binds MHC is given survival signals and moves on to the next round. T cells that don't bind MHC tightly enough die by apoptosis.

Both MHC I and MHC II are shown to developing T cells during positive selection. Whichever the T cell binds first determines if it becomes a Helper or a Killer T cell.

You may have noticed that during positive selection, we said that T cells were binding to an MHC carrying a self antigen. But, we don't want our T cells to bind our own antigens! So, the next round of selection makes sure that any T cell that binds too tightly to MHC with self antigen is destroyed. T cells travel through an area where many of your self antigens are presented by professional antigen presenting cells. They must not bind strongly to a self antigen. If they do, they are eliminated.

Rarely, some cells that react to self antigen will sneak through. This results in your T cells attacking your own body in a condition called autoimmunity.

So, T cells must be able to bind strongly to MHC, but NOT bind tightly to self antigen.

ONLY T cells that do bind MHC, but don't bind self antigen tightly go on to mature.

Becoming Helpers or Killers

When T cells arrive in the thymus, they have two kinds of receptors on their cell membrane, CD4 and CD8. As described above, during positive selection, they bind either MHC I or MHC II and choose their fate. CD4 and CD8 are buddies with the T cell receptors, so they're called co-receptors.

CD4 allows a T cell to binds MHC II - that's the MHC that only dendritic cells, macrophage and B cells have. T cells that binds MHC II during positive selection will stop producing CD8, so they only express CD4 for the rest of their lives. T cells that only express CD4 are Helper T cells that will bind only MHC II. There are only 3 types of cells that express MHC II: dendritic cells, macrophages and B cells (the professional antigen presenting cells). This means Helper T cells only get information about germs from professional antigen presenting cells.

Learn more about professional APCs here >>

CD8 allows a T cell to bind MHC I - the MHC that almost all of your cells have. A T cell that has CD8 becomes a Killer T cell. This means that they can get information from your cells about whether they are healthy or infected.

When T cells are developing, whichever MHC they bind first wins! If the T cell binds an MHC I with its CD8 co-receptor, then the T cell stops making CD4 and goes on to be a cytotoxic T cell.

Tolerance

Our immune cells learn to be tolerant to what belongs to the body. When this mechanism goes wrong, and the immune system starts an attack on itself, it is called an auto-immune disease. Our bodies can also acquire tolerance, when the body learns to tolerate external substances or cells, like in the case of a pregnant mother whose immune system must tolerate her growing baby.