What's a Receptor?
One of the most important types of immune cells are T cells. T cells have receptors. A receptor is a bump on the cell with a special shape. This shape can match with a shape on another cell - like two puzzle pieces. When a receptor finds a match, they stick together! If a T cell finds a match, that means there is a germ in the body! The T cell will then make the other immune cells excited to fight the germ!
Each T cell has its own special shape, and that shape can only bind to one piece of germ!
How Do T Cells Find Their Match?
T cells aren't the only cells with special receptors. All of your cells have their own kinds of receptors! B cells, macrophages and dendritic cells are swallowing cells with a receptor called MHC. Remember, B cells, macrophage and dendritic cells are swallowing cells that chew up germs and show the bits to T cells. These cells put the bits into their MHC receptor and try to match it with T cells. When they make a match with a T cell, they get excited and fight the disease!
Learn more about swallowing cells >>
Learn more about where T cells come from >>
A T cell (aqua) uses its receptor to match with an MHC (green) on a macrophage.
The little green squiggle in the middle is a chewed up piece of germ.
T cells come into contact with potential threats in two situations:
1 - When a Killer T cell binds to a cell that belong to the body to see if it's healthy
and
In both cases, they must first bind to another cell to communicate with it.
How does the body find infected cells?
1 - Binding to Cells that Belong to the Body - The Role of MHC I
Germs like viruses often live inside our cells; so how can our immune system find out if a cell is infected? To make the immune cells’ jobs easier, (almost) all cells of the human body have a special group of proteins on their cell surface called the Major Histocompatability Complex class I. That's quite a mouthful, so we just call it MHC I.
Cells are in a constant state of breaking down old proteins and making new proteins. MHC I captures little bits of broken-down proteins inside the cell, then travels to the surface of the cell and presents these pieces (called antigens) to other cells - like Killer T cells. All T cells have a T cell receptor that can match to ONE antigen, but Killer T cells have special receptor called CD8 that ONLY matches MHC I. This allows them to easily “dock” or bind to (almost) any cell in the body by the MHC I complex. If the little piece of protein on MHC I is from a virus and not from you, the Killer T cell will know that the cell is infected.
Watch a Killer T cell (TK) match its receptor and CD8 to the MHC I of a potentially infected cell.
How does the body find germs that live outside our cells?
2 – Binding to a Foreign Particle or Cell - The Role of MHC II
What about cells and particles that don’t belong to the body and don’t have MHCs to present their antigens? This is where swallowing cells like macrophages and other professional antigen-presenting cells come in and help the T cells!
Let’s use bacteria as an example. Once a bacterium enters the body, it will most likely be gobbled up by a phagocytic (phago means "to eat") white blood cell, like a macrophage.
The bacteria that the macrophage eats will be made of proteins that are different than the proteins made by other organisms. The macrophage eats the bacteria and breaks it down (digests it). Then, another type of Major Histocompatability Complex - this time class II (MHC II) - picks up the pieces and travels to the surface of the phagocytic cell! As you can see, MHC II has a job very similar to MHC I, but the big difference is that only 3 types of cells have MHC II, while all nucleated cells have MHC I.
Watch a Helper T cell (TH) use its receptor and CD4 to bind to an MHC II on a macrophage.
Which 3 cell types have MHC II? Macrophages, Dendritic Cells and B cells are the cells that present foreign particles (antigens) to other cells in the body and are (not so cleverly) known as the professional antigen presenting cells, or APCs.
Who do antigen presenting cells present their antigens to? Helper T cells have special receptors called CD4 that ONLY bind MHC II, so they can check out the antigen. If the Helper T cell matches the antigen, the T cell will become activated and start an immune response to get rid of the bacteria.
A macrophage is one of 3 types of phagocytic cells that have BOTH MHC I and MHC II on their surfaces and can talk to both Helper and Killer T cells.
Helper T cells and Killer T cells bind different MHCs
Helper T cells and Killer T cells recognize different types of MHC complexes; Helper T cells recognize MHC I, while Killer T cells recognize MHC II.
All of your nucleated cells only display MHC I, and therefore can bind with Killer T cells, but not Helper T cells. The professional APC's actually display antigens in both MHC I and II, so they can bind with both Killer T cells and with Helper T cells. Why?
Macrophages and T cells are very complex cells with many binding sites on their surfaces. Remember when we said the macrophage digested the bacteria and displayed bits of it on its cell surface using MHC II. The macrophage doesn’t just display one bit; it displays many bits in many different MHC II molecules all over its surface. That way it can attract as many T cells—both Helper and Killer—as possible.
When the T cells are maturing, the kind of MHC complex the T cell binds helps determine its fate as either Killer or Helper T cell.
Learn more about T cell development >>
MHC allows the T cells to bind strongly to the target cell and give them time to identify the antigen and recruit other cells.
How do the T cells identify the antigen? >>
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