1. The Immune System

2. Defenses Against Infection
Pathogens: Viruses, bacteria, and other microorganisms that cause disease.
Pathogens are present everywhere in the environment
In the food you eat
In the water you drink
In the air you breath.

3. Your Body has Several Effective Defenses Against Pathogens
The immune system carries out a major part of the protective function of blood. Cells of the immune system are on constant patrol, ready to attack foreign invaders that get past other body defenses. The immune system is not the only infection fighting system of the body, but it is the most complex. There are basically three lines of defense against disease causing organisms.

4. First Line of Defense
Involves several kinds of physical and chemical barriers These include: skin, sweat, tears, saliva, membranes lining body passages, mucus, stomach acid, and urine. Unbroken skin, and membranes lining body passages are effective barriers to most pathogens Sweat, tears, and saliva contain chemicals that kill or inhibit some kinds of bacteria Mucus traps pathogens that are then washed away or destroyed by chemicals

5. Second Line Defenses
If pathogens get past the First Line barriers, an inflammatory response results. An inflammatory response is a reaction of the body that causes swelling, redness, warmth, and pain in the area of infection. Cells damaged by the infection release chemicals that increase blood flow to the area. Increased blood flow causes puffiness and warmth and attracts macrophages to the area. A macrophage is a large white blood cell that can ingest large numbers of bacteria.

6. Second Line Defenses
Macrophages ingest the pathogens and any damaged tissue. Eventually pus: a mixture of macrophages, dead cells, bacteria, and body fluids collects in the wound. Pus is then absorbed by the body or drained, the pathogen is destroyed, the inflammation dies down and the wound is healed. If the pathogen is a virus, healthy cells surrounding the infection produce an enzyme that blocks the reproduction of the virus.

7. Third Line Defenses
If the infection is not stopped by the inflammatory response, the pathogen is targeted for destruction by the last line of defense – the immune system. The immune system recognizes, attacks, destroys, and “remembers” each kind of pathogen or foreign substance that enters the body. This is done by producing antibodies and specialized cells that bind to and inactivate pathogens.

8. Third Line Defenses
Antibodies: Proteins in the blood that bind to help destroy pathogens in the blood. Mark the pathogen for death. Unlike the First and Second Lines of Defense, the Third Line can tell the difference between different types of pathogens. For each type of pathogen, the immune system produces antibodies or cells that are specific to that pathogen.

9. Bone marrow – produces white blood cells
The Immune Response
The immune system includes all parts of the body that are involved in the recognition and destruction of foreign materials.
Bone marrow – produces white blood cells
White blood cells – especially phagocytes and lymphocytes
Tissues of the lymphatic system: lymph nodes, tonsils, spleen (these produce white blood cells called lymphocytes).

10. Which Cells are Friends and Which are Foes?
For the immune system to work the body needs to the difference between itself (own cells and materials) and nonself (foreign cells and materials) This recognition is based on differences in certain large molecules (proteins) between one organism and another. When the body recognizes that a cell is a foreign invader it produces antibodies or special cells that bind to inactivate the invader and/or target it for destruction.

11. The Immune Response
The production of antibodies and specialized cells that bind to and inactivate foreign substances is called the immune response. Antigen: Any substance that can cause an immune response. Viruses and bacteria have substances on their outer surfaces that are antigens that stimulate an immune response. Antigens are usually proteins, but carbohydrates and nucleic acids may also be antigens.

12. Antigens and Transplants
The cells of each human contain a unique combination of antigens that no other human has. As a result tissue from one person transplanted to another will act as an antigen and trigger an immune response. An immune response to an antigen acts to destroy the antigen. That is why blood transfusions and organ transplants require the antigens to match as much as possible. Otherwise the body will attack the new tissue.

13. Keeping all the terms straight.
Some of the terms sound similar and may be confusing. Pathogen: a disease causing microorganism Antigen: a substance on the outer surface of a microorganism (a pathogen or foreign invader) that triggers an immune response. Antibody: Proteins in the blood that bind to and inactivate and/or target a cell for destruction.