Each B cell has a different antibody on its cell surface.
Each antibody will recognize a unique antigen.
Antibodies vary because they have different variable regions.
This variation leads to millions of different kinds of B cells that can recognize every possible antigen any person will ever encounter in their lives!
Unknown source; figure 18.10, Purves's Life: The Science of Biology, 7th Edition
Clonal Selection
Millions of different B cells exist, each with a different, specific antibody which they display on their cell surface.
If a person is exposed to an antigen then the B cell with the matching antibody will be stimulated to divide.
This results in a clone of cells (hence the term “clonal selection”):
Some become plasma cells.
These are called effector cells because they simply secrete the antibodies that bind to the antigen (i.e. effector antibodies).
They are antibody factories.
Some become memory cells.
These stay around to provide protection for a future exposure to this antigen.
In the future, instead of having just one B cell to respond there will be many.
Figure 18.7, Purves's Life: The Science of Biology, 7th Edition
The first time a person is exposed to an antigen, there is a lag time between exposure and the resulting antibody production.
Because of the production of memory cells after the first exposure, there are more B cells that recognize that particular antigen.
When a second exposure occurs the immune system can mount a quicker response, producing more antibodies sooner.
This provides immunity to many diseases after the first occurrence of the disease.
Figure 18.8, Purves's Life: The Science of Biology, 7th Edition
Cell-Mediated Immunity
Cell-mediated immunity uses T cells, instead of antibodies: thus, it is cell-mediated immunity.
Defends against any non-self cells, including foreign cells that have been invaded by pathogens like bacteria and viruses.
This system does not attack pathogens directly, like humoral immunity—it attacks cells that have already been attacked!
T-cell receptors interact with antigens—like antibodies do in the humoral response—but are different in key ways:
T-cell receptors are glycoproteins, not globular proteins.
While antibodies bind to an intact antigen, T-cell receptors bind to a piece of the antigen displayed on the surface of the antigen-presenting cell.
Analogy:
One element of the humoral system is like attacking soldiers in the field directly (antibodies directly attack the antigen while it is floating around in the circulatory or lymphatic system).
The cell-mediated system never directly attacks the invader; instead it only attacks the invader once the invader has taken over a cell.
This is similar to when enemy soldiers overrun a tourist hotel.
As soon as they have taken over the hotel, the hotel is attacked.
Unfortunately, the hotel has to be destroyed to destroy the invaders.
T-cell receptors recognizing antigens by pieces of them displayed on the surface of the antigen-presenting cell is like a soldier sticking a gun out of the tourist hotel.
A little piece of the invader is “displayed” on the surface of the building so it can be recognized as being invaded.
Once this piece is recognized, the building will be attacked.
Unknown sources
The Process
Recognition phase
When a body cell is invaded by a bacterium, virus, antigen, etc., the body cell will display on its surface both self markers (antigens) and non-self markers (antigens).
T cells, which are constantly patrolling the body, will realize there is a problem.
Their receptors will recognize the non-self antigens on the surface of the body cell as foreign antigens.
This indicates that the cell has been invaded.
Since tissue transplant cells and cancer cells also display these non-self antigens, the T cells will realize there is a problem with these cells also.
Response phase
When T cells recognize non-self antigens, they go through a clonal selection process like with B cells.
The T cell with the receptor for this particular antigen is selected and reproduced in large quantities.
This allows the immune system to mount a massive attack on the invaded cells.
Two types of cells are produced:
Cytotoxic T cells (killer T cells): recognize and destroy non-self cells by puncturing them, causing them to lyse.
Helper T cells: stimulate the production of cytotoxic T cells and B cells.
