Unit 2 Lesson 9.pdf

LESSON 9 How can the body control its response? Signal, target, effect In order for the body to control its response, different cells and parts of the body need to be able to communicate with each other. Communication involves sending out signals that are received by a target. The signals that cell...

LESSON 9 How can the body control its response? Signal, target, effect In order for the body to control its response, different cells and parts of the body need to be able to communicate with each other. Communication involves sending out signals that are received by a target. The signals that cells send are mostly in the form of small molecules that travel from one cell to another. Immune cells and the signal molecules they send often travel through the bloodstream to reach distant target cells. The activated target cell then responds to the signal by changing in some way or doing something. We can use the ideas of signal, target, and effect to understand some examples of systems the body uses to start and stop its immune response. Amplification At the site of a wound or infection, bacteria may be present. As you learned in the last chapter, human cells at the site of an infection are likely to be damaged. As long as they are alive, the bacteria cells are constantly producing some molecules, and the damaged human cells are constantly producing some molecules. Both types of molecules are signals to immune cells. These signal molecules may leave the site of infection and travel through the bloodstream. The signal molecules interact with receptors on white blood cells wherever they encounter those cells—at the infection site, in the bloodstream, or in lymph nodes or other parts of the body. The interaction has the effect of causing the white blood cells to change, or activate, and begin to travel to and respond to the infection. So, signals from the cells at the site of the infection can target white blood cells and affect them, causing the immune response to begin. Importantly, the white blood cells send out signal molecules that target white blood cells. So more white blood cells results in more of all the activities of white blood cells, including further increasing the number of new white blood cells. Therefore, the response amplifies as it goes on. Inhibition Turning off the immune response also involves signal molecules binding to receptors on target cells. When very high levels of some of the signal molecules involved in amplifying the immune system interact with the hypothalamus in the brain, the hypothalamus responds by sending a signal of its own to another part of the brain. The latter brain cells release special hormones into the bloodstream. Hormones are a type of signal molecule. In this case, the target cells for the hormones are cells in the adrenal glands that are found above each kidney. This hormone signal is important for turning off the immune response. When the hormones bind to their target cells in the adrenal glands, they stimulate the cells to produce and release more molecules called glucocorticoids. The glucocorticoids move through the bloodstream and bind to receptors on their target cells, the white blood cells. This binding changes what the white blood cells do. For example, they cause the cells to stop producing amplification signals—and start producing other signal molecules that are inhibitory signals. They also decrease the number of white blood cells by programming them to die. Thus, the glucocorticoids act like “brakes” on the immune response, decreasing the production of C-reactive protein as well as the number of white blood cells. Notice that the immune response not only stimulates the production of cells and molecules that are responsible for amplifying the initial response, but it also triggers the production of signal molecules (glucocorticoids) that ultimately decrease this response. The glucocorticoids produced provide feedback to the immune system that turns off the immune response. Feedback: negative and positive The concept of feedback is important for understanding how the body can control, or regulate, its activities to ultimately maintain homeostasis. Feedback simply refers to a process that is a cycle in which a later step affects (or feeds back to) the first step. One type of feedback continually increases, or amplifies, what is happening in the system; we call this positive feedback. Another type of feedback decreases, or inhibits, what is happening in the system; we call this negative feedback. Feedback mechanisms allow the body to regulate its activities and maintain homeostasis. When feedback fails Even the body’s own feedback mechanisms have a range within which they can effectively operate. Organ failure and even death can result from an infection that is too difficult for the body to fight. Patients with bad bacterial infections may go into sepsis, which refers to a hyperactive immune response, followed by an overcorrection crash resulting in a suppressed immune system.