The Adaptive Immune System
Objective 11
Explain the difference between cell-mediated and antibody-mediated immunity. Describe how antigen presentation by MHC Class I molecules activates the adaptive immune system. Describe how antigen presentation by MHC Class II molecules activates another set of responses in the adaptive immune system. Recall how B cells are recruited by T helper cells.
Adaptive immunity can be divided into cell-mediated and antibody-mediated immunity. Both mechanisms are triggered by exposure to specific antigens.
In cell-mediated immunity, T lymphocytes are activated directly against abnormal cells, such as cancer cells, transplanted tissues or infected body cells.
In antibody-mediated immunity, B lymphocytes are recruited and activated by T helper cells to become plasma B cells, which produce and secrete specific antibodies. Antibodies are found in the plasma as well as other fluids throughout the body, which is why an older name for antibody mediated immunity is humoral immunity.
Because T helper cells are the middlemen of adaptive immunity, they play a part in both cell-mediated and antibody-mediated immunity.
Lymphocytes have specificity. Remember that each lymphocyte reacts only to one of the 100,000,000 possible antigens. This means that the cell will spend its life fighting the specific antigen that activated it. For example, if a B cell recognizes a specific protein on the surface of the hepatitis B virus, it will continue to produce antibodies to that one specific protein on that one specific virus. If the cell divides to form more plasma B cells or memory B cells, the daughter cells will have the same genetic makeup of the activated parent B cell (i.e. are clones) and will retain specificity to that same antigen.
We call these identical copies of the same cell a monoclonal population, and the antibodies made by these cells are called monoclonal antibodies. Through this, you are able to have cells and antibodies that help protect you against pathogens you may not have been exposed to for years or even decades.
Antigen Presentation by Infected Cancerous Cells (MHC Class I Markers)
MHC Class I molecules are found on the surface of all nucleated cells. That’s because any cell with DNA in its nucleus is subject to having that DNA altered by a virus, or by a mutation event which triggers cancer.
In each of these cases, the change in DNA means that a different set of proteins, or unusual forms of the usual proteins, will be made by the cell. The cell’s proteosomes (Unit 5) are busy shredding all the proteins of the cell, and then small pieces of each protein made by the cell are displayed on the cell surface.
The MHC molecules then act as “billboards” to display the protein fragments.
As long as the protein fragments are the same ones the immune system has always seen, nothing much happens.
But if the protein fragments displayed on the MHC Class I billboard are weird, then the T cytotoxic cell with specificity for that par