Chapter 37 The Immune System

Infectious Diseases Section 1

Pathogens Cause Infectious Disease An infectious disease is a disease that is caused by a pathogen passed from one organism to another, disrupting homeostasis in the organism’s body – Caused by agents called pathogens (bacteria, viruses, protozoans, fungi, and parasites) – Many bacteria and protozoans are present in our everyday lives without causing infections diseases. In fact, they prevent pathogens from thriving and multiplying

Germ Theory and Koch’s Experiments The germ theory states that some microorganisms are pathogens. Scientists couldn’t prove this theory until Robert Koch developed his postulates Koch isolated bacterial anthrax from a cow that had died from the infection and then injected a living cow with the pathogen. When the cow developed the infectious disease and died he isolated the pathogen again and showed that it was the same bacteria, proving that the same bacteria had infected both cows and that it was the cause of anthrax – This lead to the creation of Koch’s Postulates – steps followed today to identify a specific pathogen as the agent of a specific disease

Koch’s Postulates Postulate 1: the suspected pathogen is isolated from every host organism infected Postulate 2: the suspected pathogen is grown in a pure culture on artificial media (culture cells for viruses) in a lab Postulate 3: the suspected pathogen grown in the pure culture must cause the disease in a healthy new host Postulate 4: the suspected pathogen is isolated from the new host, and shown to have the same characteristics as the original pathogen *There are some exceptions to these postulates

Spread of Disease For a pathogen to spread it must have both a reservoir and a way to spread – A disease reservoir is a source of the pathogen in the environment Human Reservoirs: humans are the main pathogen reservoirs that infect other humans, an individual that is symptom-free but can pass a pathogen on is called a carrier Animal Reservoirs: other animals can pass pathogen on to humans Other Reservoirs: soil, water, food, etc. – Transmission of pathogens Pathogens are transmitted in 3 ways: direct contact, indirect (through the air or touch of contaminated objects), or by vector organisms (ticks, mosquitoes, and flies)

Human Infectious Diseases DiseaseCauseAffected Organ System How Disease is Spread Strep throatBacteriumRespiratoryDroplets/direct contact TuberculosisBacteriumRespiratoryDroplets RabiesVirusNervousAnimal bite Common cold/InfluenzaVirusRespiratoryDroplets/direct contact MalariaProtozoan (vector) Blood and LiverVector (mosquito) Athlete’s footFungusSkinDirect contact or contaminated objects

Symptoms of Disease When you become ill, the disease has invaded some of the cells of your body. It then multiplies in the cells and exits by exocytosis or lysing the cell. In doing this it damages tissues or destroys cells When the invading disease is pathogenic, harmful chemicals or toxins can be produced and carried throughout the body via the bloodstream and damage various parts of the body Some bacteria and protozoans and all viruses invade and live inside cells, causing damage. When cells are damaged, they might die, causing symptoms in the host – Some disease symptoms are triggered by the immune system

Disease Patterns As outbreaks happen, patterns emerge and are documented by different organizations Endemic diseases – diseases that are continually found in small amounts within the population Epidemic – diseases that have a large outbreak in a small area and afflict many people Pandemic – epidemic diseases that are widespread throughout a large region (country, continent, globe)

Treating and Fighting Diseases Doctors may prescribe drugs to help fight diseases – One type of prescription are called antibiotics (kill or inhibit the growth of microorganisms) Antibiotics only work for bacterial infections Through natural selection, antibiotic-resistant forms of bacteria have become more prevalent. They reproduce quickly and pass on their resistance. Because of this, new forms of antibiotics have to be made – This has made problems for treating such diseases, and the development of new treatments/therapies is needed

The Immune System Section 2

Nonspecific Immunity Defenses that are not aimed at a specific pathogen, they protect the body from any pathogen that it encounters – The first line of defense! Barriers: used to protect against pathogens, found in areas where pathogens might enter – Skin: layers of living cells, dead cells, oils, and bacteria that block and can inhibit many pathogens – Chemicals: saliva, tears, and nasal secretions contain lysozyme which breaks down bacterial cell walls and kills pathogens Mucus prevents bacteria from sticking to the linings of cells and cilia helps to move the bacteria back out of any passageways Hydrochloric acid secreted from the stomach kills microorganisms in your stomach

Nonspecific Immunity Nonspecific responses to invasion: the body has nonspecific immune responses to pathogens that get beyond its barriers – Cellular defense: if foreign microorganisms enter the body, immune cells defend the body. One way is by phagocytosis: a cell engulfs the,microorganism and then releases digestive enzymes and harmful chemicals, destroying the microorganism – Complement proteins aid in the process of phagocytosis by helping the phagocytic cells bind the pathogen or by forming a pore in the pathogen that aids in its destruction Type of CellFunction Neutrophils Phagocytosis: blood cells that ingest bacteria Macrophages Phagocytosis: blood cells that ingest bacteria and remove dead neutrophils and other debris Lymphocytes Specific immunity (antibodies and killing of pathogens) blood cells that produce antibodies and other chemicals

Nonspecific Immunity Nonspecific responses to invasion: the body has nonspecific immune responses to pathogens that get beyond its barriers – Interferon: when viruses enter the body they secrete a protein called interferon. This protein binds to neighboring cells and causes them to produce antiviral proteins, which can prevent viral replication – Inflammatory Responses: when pathogens damage tissues, the invader and host cells release chemicals. These chemicals attract phagocytes, increase blood flow to the area, and make blood vessels more permeable to allow WBCs to reach the area.

Specific Immunity Second line of defense when pathogens get past the nonspecific immunity responses More effective, but takes time. Involves tissues and organs from lymphatic system Lymphatic system: filters lymph (fluid that leaks out of capillaries to bathe body cells) and blood, destroys foreign microorganisms, and absorbs fat Lymphatic organs: – Contains lymphatic tissue, lymphocytes, other cell types, and connective tissue – Includes: Lymph nodes (filter lymph and remove foreign materials), Tonsils, Spleen (stores blood and destroys damaged RBCs), Thymus gland (activates T cells), and Various tissues from the intestine, respiratory, urinary, and genital tracts

B Cell and T Cell Responses Antibodies are proteins produced by B lymphocytes (B cells) that specifically react with antigens – Antigens are substances foreign to the body that cause immune responses A.K.A. – “antibody factories” Steps of B cells creating antibodies: 1.Pathogens are engulfed by macrophages and the macrophage presents a small portion of the pathogen outside (called the processed antigen) 2.The processed antigen is taken by a helper T cell which then reproduces 3.The processed antigen is given to the B cell (via the helper T cell) and now the B cell is activated 4.The activated B cell divides and the resulting B cells either produce antibodies or become memory B cells for future pathogen invasion When helper T cells are activated by processed antigens they can also bind to cytotoxic T cells which either destroy pathogens by attaching to them and making them lyse or by releasing chemicals called cytokines – Cytokines cause immune system cells to divide and recruit immune cells to the area of infection

Passive and Active Immunity One result of the specific immune response is memory B and T cells – These long-living cells remember the pathogen from infection and are ready to fight if the pathogen returns Passive immunity: occurs when antibodies are made by other people or animals and are transferred or injected into the body – Mother to fetus via placenta – Mother to infant via breast milk – People exposed to hepatitis A/B, tetanus, rabies, and snake and scorpion venom via injection Active immunity: occurs after the immune system is exposed to disease antigens through infection or immunization (deliberate exposure to develop memory cells)

Immune System Failure Defects and diseases in the immune system can result in the increased likelihood of developing infectious diseases – AIDS is an immune disorder that is caused by a virus (HIV) HIV infects helper T cells, causing them to produce new HIV viruses and infect other helper T cells. Eventually, there are less helper T cells to fight and the virus takes over