STEP 1 - How T Cells Bind to the Target Cell: the MHC protein complex
T cells come into contact with 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 the cell in question.
1 - Binding to Cells that Belong to the Body - The Role of MHC I
To make the immune cells’ job easier, (almost) all cells of the human body have a special group of proteins called the Major Histocompatability Complex class I – the MHC I.
Cells display proteins (antigens) on their membrane with MHC I. The MHC I captures proteins inside the cell and presents them to the outside. For this reason, body cells are called antigen-presenting cells (APCs).
Killer T cells have a special binding site that matches the MHC class I. This allows them to easily “dock” or bind to (almost) any cell of the body by the MHC class I complex.
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 - 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 white blood cell, like a macrophage.
The bacteria that the macrophage eats contains its own specific DNA, and this DNA is different from the DNA of any other type of bacteria. The macrophage eats the bacteria and breaks it down (digests it), coating it in proteins. These proteins are the Major Histocompatability Complex II (MHC II)! The macrophage takes the MHC II-covered bits of bacteria and displays them on its own cell membrane. In fact it is the MHC II proteins’ job to take the germ proteins to the membrane of the cell.Helper T cells have special binding sites that bind the MHC II, so they can access the antigen.
Macrophages and T cells are very complex cells with many binding sites on their surfaces. Think of when the macrophage digested the bacteria and displayed bits of it on its cell surface surrounded by the special MHC protein. The macrophage doesn’t just display one bit; it displays many molecules all over its surface. That way it can attract as many T cells—both Helper and Killer—as possible.
Helper T cells and Killer T cells bind different MHCs
Helper T cells and Killer T cells don’t recognize the same MHC protein complexes. Helper T cells recognize MHC I, while Killer T cells recognize MHC II.
Regular APC's (normal body cells) only display MHC I, and so 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 Killer T cells and with Helper T cells.What kind of MHC complex the T cell binds helps determe its fate as a Killer or Helper T cell. This decision occurs during the maturation of the T cell >>
MHC allows the T cells to bind strongly to the target cell and give them time to identify the antigen itself and recruit other cells.
How do the T cells identify the antigen? >>
