Immune System for AP Biology
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.
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