THE LIVING WORLD
Unit Six. Animal Life
27. How the Animal Body Defends Itself
To understand how this third line of defense works, imagine you have just come down with the flu. Influenza viruses enter your body in small water droplets inhaled into your respiratory system. If they avoid becoming ensnared in the mucus lining the respiratory membranes (first line of defense), and avoid consumption by macrophages (second line of defense), the viruses infect and kill mucous membrane cells.
At this point, macrophages initiate the immune response. Macrophages inspect the surfaces of all cells they encounter. Every cell in the body carries special marker proteins on its surface, called major histocompatibility proteins, or MHC proteins. The MHC proteins are different for each individual, much as fingerprints are. The MHC protein on the cell in figure 27.8a is exactly the same on all cells in that person’s body. As a result, the MHC proteins on the tissue cells serve as “self’ markers that enable the individual’s immune system to distinguish its cells from foreign cells. For example, the foreign microbe in figure 27.8b has different surface proteins that are recognized as antigens.
When a foreign particle infects the body, it is taken in by cells and partially digested. Within the cells, the viral antigens are processed and moved to the surface of the plasma membrane, as shown in figure 27.8c. The cells that perform this function are called antigen-presenting cells and are usually macrophages. At the membrane, the processed antigens are complexed with the MHC proteins. This process is critical for the function of T cells because T cell receptors can be called into action only when antigens are presented in this way. B cells can interact with free antigens directly.
Figure 27.8. How antigens are presented.
(a) Cells of the body have MHC proteins on their surfaces that identify them as "self" cells. Immune system cells do not attack these cells. (b) Foreign cells or microbes have antigens on their surfaces. (c) T cells can bind to the antigens to initiate an attack only after the antigens are processed and complexed with MHC proteins on the surface of an antigen-presenting cell. B cells recognize the antigens directly, not requiring an antigen-presenting cell. (d) In this electron micrograph, a lymphocyte (right) contacts a macrophage (left), an antigen-presenting cell.
Macrophages that encounter pathogens—either a foreign cell such as a bacterial cell, which lacks proper MHC proteins, or a virus-infected body cell with telltale viral proteins stuck to its surface—respond by secreting a chemical alarm signal. The alarm signal is a protein called interleukin-1 (Latin for “between white blood cells”). This protein stimulates helper T cells. The helper T cells respond to the interleukin-1 alarm by simultaneously initiating two different parallel lines of immune system defense: the cellular immune response carried out by T cells and the antibody or humoral response carried out by B cells. The i mmune response carried out by T cells is called the cellular response because the T lymphocytes attack the cells that carry antigens. The B cell response is called the humoral response because antibodies are secreted into the blood and body fluids (humor refers to a body fluid).
Key Learning Outcome 27.4. When macrophages encounter cells without the proper MHC proteins, they secrete a chemical alarm that initiates the immune defense.