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Second Line Of Defense

The first line of defense is comprised of two barriers, the physical and chemical barriers. These barriers or blockades are always ready and prepared to defend the body from infection. The first barrier is the physical blockade between the inside of the body and the outer world. Physical blockades of the first defense system includes the skin, tears, and the stomach. After the outer line of defense is breached, the body starts it innate immune system to help prepare to take action on the foreign infectious disease or bacteria.

The chemical barrier or the innate immunity is made up of a collection of white blood cells along with groups of chemical messengers. Innate immunity is activated by a set of molecules that are found only on invading substances rather than on cells of the body. Any sign of bacterial lipopolysaccharide, double-stranded RNA and bacterial flagellin will trigger an inflammatory response. These barriers include skin, which forms a waterproof mechanical barrier, and tears, mucus, and saliva, that have an enzyme which destroys the cell wall of many bacteria.

Very thin tiny hairs or cilia help filter the bacteria. The stomach acid kills bacteria in a person’s stomach, then the urine flow flushes out pathogens in people’s urine. The beneficial bacteria grows in many places like in a person’s mouth which helps kill bacteria. Neutrophils or certain white blood cells help kill and ingest pathogens trying to enter into the body. Pathogenic microorganisms or infections that pass the first line of defense will go through the second line of defense. Second Line of Defense

The second line of defence is a group of cells, tissues, and organs that work together to protect the body. This defense system is non-specific and non-adaptive. In the second line there are many cells involved such as leukocytes, neutrophils, T helper cells, cytotoxic T-cells, macrophages, dendritic cells, B-cells, and suppressor T-cells. The phagocytes or neutrophil squeeze through the capillary wall and into the infected tissue to engulf and digest offending bacteria. The bacteria will be attracted to the membrane of the neutrophil. Phagocytosis.

The neutrophil will engulf the bacteria. Once in the neutrophil, lysosomes or vesicles containing digestive enzymes, will form and make their way towards the phagosome containing the bacteria. The lysosomes will fuse with the phagosome. Now the bacteria will be killed and digested by enzymes. There are also a couple of tissues and organs involved in this line of defense. These are the lymphatic system, the lymph nodes and lymph fluid. The cells, tissues and organs form the body’s second line of defence against pathogenic microorganisms or more commonly known as infection.

They are responsible for the body’s ability to fight against infections and to protect people and their body. By working together all theses cells help people maintain a homeostatic balance of their immune system. There are four steps that describe the inflammation of the second defense system. When cells are injured they release distress signals/chemicals which initiate inflammation. This includes histamine release by basophils/mast cells. Capillaries in the involved area dilate and become leaky. Dilated capillaries mean increased blood flow to the area.

Leaky capillaries mean easier entry for WBCs and important blood proteins like antibodies. Inflammatory signals and swelling activate nearby pain receptors. Phagocytes, neutrophils, and monocytes are attracted to the area and start working to phagocytize pathogens and damaged and dead tissue cells. Third Line of Defense The third line of defense is a specific resistance or acquired immunity. The composition of this immune defense relies on antigens, which are specific substances found in foreign microbes. This line of defense all depends on the lymphocytes.

The diagram below shows the pathway of the third immune response or adaptive immune response. There are two types of lymphocytes that are made in the bone marrow. The first is the T-cell which migrates from the bone marrow to the thymus. The other cells is the B-cell which also matures in the bone marrow, spleen, or foetal liver. Once these cells mature they then move around the body hunting for foreign antigens. The two cells have different responses, the T-cells are involved in a cell-mediated response, while the B-cell are involved in the humoral response.

During the process of the T-cell response there are many different variations of the T-cell such as the killer T-cell, helper T-cells, suppressor T-cells, and memory cells. By using cytotoxic T-cell, the third response is able to detect and recognize antigens on the surface of infected cell. The cytotoxic T-cells then bind to the infected cells secrete cytotoxins that induce apoptosis in the infected cell and perforins that cause perforations in the infected cells. When both of these mechanisms are through they will help destroy pathogens in the infected body.

This diagram will help with the visualization of these mechanism and their processes. Killer T-cells combine with antigens on the surface of any invading cell and release a powerful group of chemicals called lymphokines. Some lymphokines kill the pathogens directly, others stimulate lymphocytes to become active and help increase the inflammation so that there are more macrophages. Helper T-cells co-operate with B-cells in the antibody production. They also activate macrophages and promote inflammation.

Memory cells remain even after the pathogens have been killed to stop re-infection. In the humoral response the T-cell and the B-cell will form a clone if it comes into contact with a complementary shaped antigen. The cell clones consists mostly of plasma cells for immediate use and some memory cells for use in the future. Plasma cells can produce several thousand antibody molecules in a second. Unlike the T-cells, the B-cells do not leave the lymph nodes, only the protein molecules that move around the body.

These proteins are released into the blood and carried to the area of infection to help prevent an infection. Only the right shaped cell will bind with an appropriate antigen or one that stated the infection. The antibody molecule, on the other hand, binds to the antigen in a similar way to a substrate binding with an enzyme. The fit, however is not as precise as the enzyme-substrate complex. The better the fit, the stronger the subsequent immune response will be. When combining the antibody molecules and the antigens together, the pathogens label these new structures as foreign.

As for tissue transplants this defense system means that the recipients of organ transplants are not assured to have an exact match. The third defense system will attack any cell, structure, or anything foreign to a person’s body because it sees it as an infection. The only way doctors have been able to successfully transplant an organ or tissue is to give the recipient immuno-suppressant drugs to destroy the T-cells in that person’s body. The diagram provided describes the process of antibody and antigen complexes forming.

There are steps that provide even greater depth into the process of these two structures and their mergence. The pathogens clumping together make them more vulnerable to phagocytes. The antibody in a sense tag the bacteria, when the bacteria sticks it makes it easier to recognise the phagocytes. Any antigens acting as toxins in a person’s body are neutralised when the antibody sticks to it. An example of this last step is comparable to an antibodies that act as antitoxins. In a similar way, if a virus has an antibody attached to it, it will no longer be able to attach or enter a host cell.

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