Immune cells continuously patrol the body, searching, ready to protect and defend. They patrol the body by traveling in the blood through the circulatory system hunting for and destroying pathogens. Many different immune cells are involved in the events in the battle of the immune response. The immune system is a complex multifaceted system made up of a variety of cells with specific jobs that work together in their tiered response to infection. This tiered defense system has a three tiered attacks that can escalate in its responses. The first line of defense focuses on keeping pathogens out of the body. The second line of defense kicks in when the first line of defense fails and pathogens enter the body. The third line of defense is the immune response, calling in the forces of both T-cells and B-cells. Each of the defensive responses employed by the immune system, relates directly to the battle status between the intruding pathogens and the body. To summarize, the immune response is a brilliant multifaceted, escalating system of organized methodical attacks, which is able to immobilize most pathogens.
First Line of Defense
The central purpose of the immune system's first line of defense is to keep invaders out of the body. The immune system protects our body by blocking access to and guarding all points of entry. In this way, many pathogens are denied entrance into our bodies and are destroyed before they are even noticed.
The largest organ of our body
our skin is like a wall or barrier covering our body and protecting it in the most fundamental way. There are three reasons why the environment on our skin inhibits the growth of or kills bacterial pathogens. One way the skin inhibits growth of bacteria is because it has a low PH. Another way the skin inhibits growth of bacteria is because the skin is coated with sweat and chemicals that kill bacteria. The third reason that the skin inhibits growth of bacteria on the skin is because the skin is covered with a thick coating of normal good bacterial flora, which crowds out pathogens. Only when the skin is opened by a wound can these pathogens enter. In addition to these three protections, the first line of defense involves guarding the gates of entry. These points of entry or gates are the mouth, the stomach, the nose, and the eyes. At each of these points, the guards serve to expel or destroy pathogens. Invaders are either expelled or destroyed by fluids, or are removed by reflexive actions, like coughs or sneezes, at the gates. One fluid used to guard the gates is mucus. The nose and throat are covered with a sticky mucus membrane that contains antibacterial enzymes which trap and kill pathogens before they are able to enter the lungs. Another fluid that guards the gates of entry is our tears. Our eyes secrete tears that contain an enzyme called lysozyme, which kills bacteria and washes pathogens away. Furthermore, saliva, an acidic fluid composed of destructive chemicals that prevents microbes from growing , guards the entrance of the mouth. Moreover, the stomach is filled with acidic fluids and enzymes that destroy most pathogens if they are ingested . Additionally, reflexive actions like coughing and sneezing will expel pathogens. In conclusion, the central purpose of the body's first line of defense is to provide a strong barrier that will keep out the pathogens or intruders and to guard all gates, or points of entry.
Second Line of Defense: Inflammatory Response
The inflammatory response is the immune system's second line of defense. The primary purpose of the inflammatory response is to get situations under control when the first line of defense fails. This inflammatory response kicks in when pathogens break through the body's barriers and enter the body from a cut, insect bite, or injury. The sequence of events is the same for the entry of all pathogens. However, the speed of the sequence can vary.
The first event of this sequence begins when pathogens begin to damage cells. When this occurs, the damaged cells send out chemical signals to "bring in the troops", or white blood cells to the site. Because of this chemical response, blood vessels widen resulting in an increased blood flow to the area allowing large numbers of phagocytes to be transported to the site. In this way, phagocytes, a type of white blood cells, respond to this signal and are transported to the area, flooding the site. Once the phagocytes arrive at the site, they begin the fight by eating anything that is not part of our body. The phagocytes are eating machines. They begin to engulf, digest, and destroy the pathogens. This process is called phagocytosis. In addition to phagocytosis, the inflammatory response uses heat and swelling to battle pathogens. The widen blood vessels and leaking fluid makes the area red, swollen, and warm to the touch. Chemicals released, as part of their immune reaction, also increase body temperature. As a result, the growth of pathogens slows down.
Third Line of Defense: The Specific and Adaptive Immune Response
The third line of defense is activated when the second line of defense breaks down and the pathogens are outnumbering the phagocytes' ability to contain, control, and suppress the growth of pathogens. As a result of this break down, the infection becomes severe enough to cause the body to develops a fever. This fever then activates the third line of defense which is called the specific immune response. This third line of defense acts like calling in the "Special Forces." This defense is waged on two main fronts, and involves cells that are highly trained experts with specialized abilities.
In responses to an invasion, or pathogen entering the body, the immune system begins to activate a cascade of multiple processes. This process begins when the macrophages, the patrolling cells, find an invader, or antigen. The macrophage cells engulf and digest the antigen, breaking it up into tiny pieces. Then, the macrophage displays this tiny piece of antigen on its cell surface. These little pieces are called antigen peptides. When an antigen is displayed on the surface of the macrophage cell it is called antigen presenting and plays an important role in the cascade of events of this immune response. Why is this antigen presenting process so important? Through "antigen presenting" the immune cells are able to communicate important information to each other. The antigen presented on the surface of the macrophage communicates the exact shape of the specific invader, or pathogen, to the Helper T-cell. This communication provides essential information needed for the ensuing level of attack.
Thus, the next key event in this sequential immune responses is the activation of Helper T- cells. Helper T- cells are activated (CD4) when they connect to the antigen presenting macrophage. During this connection, the macrophage releases a chemical called interleukin -1, which sends out a distress signal and stimulates the T- helper cells to release a chemical called interleukin -2, which initiates the next level response from the T-cells and B-cells. Also, while the Helper T cell is connected to the antigen presented on the macrophage, it physically senses the exact shape of the antigen. This communication allows the Helper T- cells to make their own protein receptors that will recognize the specific pathogens communicated by the antigen presenting macrophage. The Helper T- cell then displays these protein receptors on their cell surface. In this way, the T helper cell is then able to communicate this vital identifying information to the T-cells and B-cells. In this way, the Helper T-cell activates the next level of response.
Communicating this information, the Helper T- cells activate a parallel response, involving the initiation both the humoral and the cell- mediated immune response. The humoral immune response involves the B- cells finding and killing antigens in the fluids of our body. The cell-mediated response involves the Killer T-cells that will destroy the cells that have already been infected by the antigen. In the initiation of the humoral response, the Helper T - cells communicate the shape of the specific antigen to the B-cells. With this information, the B-cells are able to make an infinite number of antibodies specific to the antigen. In fact, the B- cell becomes a large plasma cell factory that produces identical copies of specific antibody molecules at an astonishing pace--up to 10 million copies an hour."
In addition, the B-cells clone themselves creating a fleet of fighters. When an antigen is found, B-cell release antibodies that clump onto the pathogen . At the same time, the Helper T- cells activate macrophages that will then be able to go out in the body's fluids, to patrol and kill the specific antigens in the humoral regions, or fluids filled areas of our body.
Simultaneously, the Helper T- cells activate the cell- mediated immune response. The cell mediated response creates and activates Killer T- cells. These Killer T- cells clone themselves to produce many Killer T- cells. Then the Killer T- cells aggressively patrol the body searching for infected cells. They carry T-cell receptors (TCR) which are super sensitive, on their cell surface that will allow them to recognize a specific antigens on an infected cell surface. When a cell is infected, the fragments, or antigens, from the infecting pathogen are broken down and transported to the surface of the infected cell. These fragments are displayed on the cell surface as peptide antigens. The antigen is presented to the T- cell by a molecule called major histocompatibility complex (MHC).
If the T cell latches onto the peptide antigen, and fits, it signals the T-cell to destroy the infected cell. As a result, viruses within the host cell are destroyed along with the infected cell thus preventing further replicating.
Both the Killer T- cell and B -cells have super sensitive receptors on their surfaces, like little hands, that are able to recognize pathogens. However, the T cell receptors stay on the cell, while B cell receptors, or antibodies, can detach from the cell. The antibodies when released from the B cells bind to the antigens they find, fitting perfectly, like a lock and key or a handshake. In this way, the antibodies are able to neutralize pathogens. As the antibodies bind to the antigens, they begin to clump together which then flags the clumps for destruction by macrophages. Finally, some T- cells and B- cells hold onto the antigen information and become memory cells, patrolling the body remembering the specific pathogen, ready to destroy it on sight for several decades.