Immune System for AP Biology (page 2)

By — McGraw-Hill Professional
Updated on Oct 24, 2011

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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