Immune System for AP Biology (page 2)
Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology
What we are about to witness is an absolute treat. We just got word from the central office that the body we are touring has just received a vaccination. A vaccine is given to a patient in an effort to prime the immune system for a fight against a specific invader. This truly is a rare opportunity for us to see the immune system in action.
We have reentered the general bloodstream circulation of the body in an attempt to find some activity. While we are in transit, I will explain some basic immune system terms to you.
The immune system is a two-tiered defense mechanism. It consists of nonspecific immunity and specific immunity. Nonspecific immunity is exactly how it sounds—it is the nonspecific prevention of the entrance of invaders into the body. Saliva contains an enzyme called lysozyme that can kill germs before they have a chance to take hold. Lysozyme is also present in our tears, providing a nonspecific defense mechanism for our eyes. The skin covering the entire body is a nonspecific defense mechanism—it acts as a physical barrier to infection. The mucous lining of our trachea and lungs prevent bacteria from entering cells and actually assists in the expulsion of bacteria by ushering the bacteria up and out with a cough. Finally, remember how I told you that you did not want to get out of the bus in the stomach? That is also the case for bacteria—it is a dangerous place for them as well. The acidity of the stomach can wipe out a lot of potential invaders.
A nonspecific cellular defense mechanism is headed up by cells called phagocytes. These cells, macrophages and neutrophils, roam the body in search of bacteria and dead cells to engulf and clear away. Some assistance is offered to their cause by a protein molecule called complement. This protein makes sure that molecules to be cleared have some sort of identification displaying the need for phagocyte assistance. Complement coats these cells, stimulating phagocytes to ingest them. Cells involved in mechanisms that need cleanup assistance, such as platelets, have the ability to secrete chemicals that attract macrophages and neutrophils to places such as infection sites to help in the elimination of the foreign bacteria. They are nonspecific because they are not seeking out particular garbage … they are just looking for something to eat.
A prime example of a nonspecific cellular response is inflammation. Let's say that you pick up a tiny splinter as you grab a piece of wood. Within our tissues lie cells known as mast cells. These cells contain the signal histamine that calls in the cavalry and initiates the inflammation response. Entrance of the splinter damages these mast cells, causing them to release histamine, which migrates through the tissue toward the bloodstream. Histamine causes increased permeability and bloodflow to the injured tissue. The splinter also causes the release of signals that call in our nonspecific phagocytic cell friends, which come to the site of the injury to clear away any debris or pathogens within the tissue. The redness and warmth associated with inflammation occur because of the increase in bloodflow to the area that occurs in this process.
The immune system also contains defense mechanisms, which are quite specific. One such defense mechanism involves white blood cells, also called lymphocytes. There are two main flavors of lymphocytes: B cells and T cells. These cells are made in the bone marrow of the body and come from cells called stem cells. B cells mature in the bone marrow, and T cells mature in the thymus. The two main types of B cells are plasma cells and memory B cells, and the two main types of T cells are helper T cells and cytotoxic T cells. Cytotoxic T cells are the main players involved in cell-mediated immunity. Helper T cells, which assist in the activation of B cells, recognize foreign antigens on the surface of phagocytic cells and bind to these cells. After binding, they multiply to produce a bunch of T cells that pump out chemical signals, which bring in the B cells to respond.
We have arrived at the vaccination site in the left arm, and things are definitely heating up here. An antigen is a molecule that is foreign to our bodies and causes the immune system to respond. What is occurring right now is the process called the primary immune response. Every B cell has a specific (randomly generated) antigen recognition site on its surface. B cells patrol the body looking for a particular invader. When a B cell meets and attaches to the appropriate antigen, it becomes activated, and the B cell undergoes mitosis and differentiation into the two types of cells mentioned earlier: plasma cells and memory cells. The plasma cells are the factories that produce antibodies that function in the elimination of any cell containing on its surface the antigen that it has been summoned to kill. These antibodies, when released, bind to the antigens, immobilizing them and marking them for the macrophages to engulf and eliminate. This type of immune response falls under the category of humoral immunity—immunity involving antibodies.
Someone had a question? How do antibodies recognize the antigen they are designed for? Excellent question. Antibodies are protein molecules with two functional regions. One end is called the fragment antigen binding region or Fab—this is what allows an antibody to recognize a specific antigen. It is designed by the plasma cell to have an Fab that binds to the antigen of interest. The other end, which binds to effector cells, is called the Fc region. There are five types of Fc regions, one for each of the five types of antibodies: IgA, IgD, IgE, IgM, and IgG. Each antibody type serves a slightly different function and is present in different areas of the body. When the antibodies bind to an antigen, complement gets involved, and this combination of antibodies and complement leads to the elimination of the invader.
I see a hand raised in the back. Yes, you are correct that I neglected to mention the memory cells. Very good. Memory cells contain the basis for the body's secondary immune response to invaders. Memory cells are stored instructions on how to handle a particular invader. When an invader returns to our body, the memory cells recognize it, produce antibodies in rapid succession, and eliminate the invader very quickly. The secondary immune response is much more efficient than the primary response. This is why few people are infected by sicknesses such as chickenpox after they have had them once already—their memory cells protect them. One important fact that does come up on the exam is that the secondary immune response produces a much larger concentration of antibodies than does the primary response.
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