EONS LEARNING

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      • Study Skills
      • Periodic Table
      • Common Ion Sheet
      • The Scientific Method
      • Doing Background Research
    • Introduction and Course Philosophy
    • Unit 1: Beginning Chemistry >
      • Lesson 1: The Atom >
        • What is an Atom?
        • The Structure of Atoms
        • The Periodic Table
        • Modeling Atoms
      • Lesson 2: Chemical Bonding >
        • Why Do Atoms Form Bonds?
        • Ionic Bonding
        • Covalent Bonding
        • Intermolecular Attraction
      • Lesson 3: Chemical Nomenclature >
        • Octet Rule
        • Ionic Compounds
        • Covalent Molecules
      • Lesson 4: Molecular Molecules >
        • Modeling Ionic Compounds
        • Modeling Covalent Molecules
      • Lesson 5: States of Matter >
        • States of Matter
        • Phase Changes
      • Lesson 6: Density >
        • What is Density?
        • Calculating Density
      • Lesson 7: Thermodynamics >
        • Temperature
        • Heat
        • Gas Laws
      • Lesson 8: Solution Chemistry >
        • Diffusion
        • Solutions and Molarity
        • Semi-Permeable Membranes
      • Lesson 9: Thermal Expansion >
        • Thermal Expansion
    • Unit 2: Earth Science >
      • Lesson 10: Earth at a Glance >
        • Perspective
        • Maps
      • Lesson 11: Layers of the Earth >
        • Layers of the Earth
      • Lesson 12: Plate Tectonics >
        • Plate Tectonics
      • Lesson 13: Rocks and Minerals >
        • Rocks and Minerals
      • Lesson 14: Particle Sorting >
        • Differentiation
        • Deposition of Sediment
      • Lesson 15: The Atmosphere >
        • Composition of the Atmosphere
        • Layers of the Atmosphere
        • Change Over Time
        • Atmospheric Disruption
    • Unit 3: The Cell >
      • Lesson 16: Life >
        • What is Life?
        • Structural Hierarchy of Living Things
      • Lesson 17: Biochemistry >
        • Intro to Biochemistry
        • Water
        • Micromolecules
        • Energy, Carbohydrates, Lipids
        • Protein and Nucleic Acid
      • Lesson 18: Cells >
        • What are Cells?
        • Microscopy
        • Plant and Animal Cells
      • Lesson 19: Membrane Transport >
        • A Special Environment
        • The Structure of Membranes
        • Membrane Transport
      • Lesson 20: Energy and Cell Respiration >
        • Energy in Biology
        • Energy Diagrams
        • Glycolysis and Anaerobic Respiration
        • Aerobic Cellular Respiration
      • Lesson 21: Photosynthesis >
        • Plants Get Energy From The Sun
        • Photosynthesis Process
        • Energy, Ecosystems, and the Environment
    • Unit 4: Anatomy and Physiology >
      • Lesson 22: The Human Body >
        • What Are Bodies Made Of?
        • What Do Bodies Do?
      • Lesson 23: The Nervous System >
        • The Nervous System
        • Neuronal Communication
        • The Central Nervous System
      • Lesson 24: The Endocrine System >
        • The Endocrine System
        • Hormones
        • Hormones, Puberty, and Reproduction
      • Lesson 25: The Integumentary System >
        • The Integumentary System
      • Lesson 26: The Musculoskeletal System >
        • The Skeletal System
        • The Muscular System
        • Anatomy Of The Musculoskeletal System
      • Lesson 27: The Cardiovascular System >
        • Blood and Blood Vessels
        • The Heart
      • Lesson 28: The Respiratory System >
        • The Respiratory System
      • Lesson 29: The Digestive System >
        • The Digestive System
        • Nutrition
      • Lesson 30: The Excretory System >
        • The Excretory System
      • Lesson 31: The Immune System >
        • Disease and Infection
        • Immunity
    • Units 5-6 Coming Soon
  • Workbench
    • Unit 1 >
      • EIS >
        • Lesson 1: Atoms
        • Lesson 2: Chemical Bonding
        • Lesson 3: Chemical Nomenclature
        • Lesson 4: Molecular Models
        • Lesson 5: States of Matter
        • Lesson 6: Density
        • Lesson 7: Thermodynamics
        • Lesson 8: Solution Chemistry
        • Lesson 9: Thermal Expansion
      • Unit 1: Project
      • Unit 1: Exam Review
      • Unit 1: Exam
    • Unit 2 >
      • EIS >
        • Lesson 10: Earth at a Glance
        • Lesson 11: Layers of the Earth
        • Lesson 12: Plate Tectonics
        • Lesson 13: Rocks and Minerals
        • Lesson 14: Particle Sorting
        • Lesson 15: The Atmosphere
      • Unit 2: Project
      • Unit 2: Exam Review
      • Unit 2: Exam
    • Unit 3 >
      • EIS >
        • Lesson 16: Life
        • Lesson 17: Biochemsitry
        • Lesson 18: Cells
        • Lesson 19: Membrane Transport
        • Lesson 20: Energy and Cell Respiration
        • Lesson 21: Photosynthesis
      • Unit 3: Project
      • Unit 3: Exam Review
      • Unit 3: Exam
    • Unit 4 >
      • EIS >
        • 22: The Human Body
        • 23: The Nervous System
        • 24: The Endocrine System
        • 25: The Integumentary System
        • 26: The Musculoskeletal System
        • 27: The Cardiovascular System
        • 28: The Respiratory System
        • 29: The Digestive System
        • 30: The Excretory System
        • 31: The Immune System
      • Unit 4 Project
      • Unit 4 Exam Review
      • Unit 4 Exam
    • Units 5-6 Coming Soon
  • Donate
  • Home
  • About Us
  • Classroom
    • Resources >
      • Study Skills
      • Periodic Table
      • Common Ion Sheet
      • The Scientific Method
      • Doing Background Research
    • Introduction and Course Philosophy
    • Unit 1: Beginning Chemistry >
      • Lesson 1: The Atom >
        • What is an Atom?
        • The Structure of Atoms
        • The Periodic Table
        • Modeling Atoms
      • Lesson 2: Chemical Bonding >
        • Why Do Atoms Form Bonds?
        • Ionic Bonding
        • Covalent Bonding
        • Intermolecular Attraction
      • Lesson 3: Chemical Nomenclature >
        • Octet Rule
        • Ionic Compounds
        • Covalent Molecules
      • Lesson 4: Molecular Molecules >
        • Modeling Ionic Compounds
        • Modeling Covalent Molecules
      • Lesson 5: States of Matter >
        • States of Matter
        • Phase Changes
      • Lesson 6: Density >
        • What is Density?
        • Calculating Density
      • Lesson 7: Thermodynamics >
        • Temperature
        • Heat
        • Gas Laws
      • Lesson 8: Solution Chemistry >
        • Diffusion
        • Solutions and Molarity
        • Semi-Permeable Membranes
      • Lesson 9: Thermal Expansion >
        • Thermal Expansion
    • Unit 2: Earth Science >
      • Lesson 10: Earth at a Glance >
        • Perspective
        • Maps
      • Lesson 11: Layers of the Earth >
        • Layers of the Earth
      • Lesson 12: Plate Tectonics >
        • Plate Tectonics
      • Lesson 13: Rocks and Minerals >
        • Rocks and Minerals
      • Lesson 14: Particle Sorting >
        • Differentiation
        • Deposition of Sediment
      • Lesson 15: The Atmosphere >
        • Composition of the Atmosphere
        • Layers of the Atmosphere
        • Change Over Time
        • Atmospheric Disruption
    • Unit 3: The Cell >
      • Lesson 16: Life >
        • What is Life?
        • Structural Hierarchy of Living Things
      • Lesson 17: Biochemistry >
        • Intro to Biochemistry
        • Water
        • Micromolecules
        • Energy, Carbohydrates, Lipids
        • Protein and Nucleic Acid
      • Lesson 18: Cells >
        • What are Cells?
        • Microscopy
        • Plant and Animal Cells
      • Lesson 19: Membrane Transport >
        • A Special Environment
        • The Structure of Membranes
        • Membrane Transport
      • Lesson 20: Energy and Cell Respiration >
        • Energy in Biology
        • Energy Diagrams
        • Glycolysis and Anaerobic Respiration
        • Aerobic Cellular Respiration
      • Lesson 21: Photosynthesis >
        • Plants Get Energy From The Sun
        • Photosynthesis Process
        • Energy, Ecosystems, and the Environment
    • Unit 4: Anatomy and Physiology >
      • Lesson 22: The Human Body >
        • What Are Bodies Made Of?
        • What Do Bodies Do?
      • Lesson 23: The Nervous System >
        • The Nervous System
        • Neuronal Communication
        • The Central Nervous System
      • Lesson 24: The Endocrine System >
        • The Endocrine System
        • Hormones
        • Hormones, Puberty, and Reproduction
      • Lesson 25: The Integumentary System >
        • The Integumentary System
      • Lesson 26: The Musculoskeletal System >
        • The Skeletal System
        • The Muscular System
        • Anatomy Of The Musculoskeletal System
      • Lesson 27: The Cardiovascular System >
        • Blood and Blood Vessels
        • The Heart
      • Lesson 28: The Respiratory System >
        • The Respiratory System
      • Lesson 29: The Digestive System >
        • The Digestive System
        • Nutrition
      • Lesson 30: The Excretory System >
        • The Excretory System
      • Lesson 31: The Immune System >
        • Disease and Infection
        • Immunity
    • Units 5-6 Coming Soon
  • Workbench
    • Unit 1 >
      • EIS >
        • Lesson 1: Atoms
        • Lesson 2: Chemical Bonding
        • Lesson 3: Chemical Nomenclature
        • Lesson 4: Molecular Models
        • Lesson 5: States of Matter
        • Lesson 6: Density
        • Lesson 7: Thermodynamics
        • Lesson 8: Solution Chemistry
        • Lesson 9: Thermal Expansion
      • Unit 1: Project
      • Unit 1: Exam Review
      • Unit 1: Exam
    • Unit 2 >
      • EIS >
        • Lesson 10: Earth at a Glance
        • Lesson 11: Layers of the Earth
        • Lesson 12: Plate Tectonics
        • Lesson 13: Rocks and Minerals
        • Lesson 14: Particle Sorting
        • Lesson 15: The Atmosphere
      • Unit 2: Project
      • Unit 2: Exam Review
      • Unit 2: Exam
    • Unit 3 >
      • EIS >
        • Lesson 16: Life
        • Lesson 17: Biochemsitry
        • Lesson 18: Cells
        • Lesson 19: Membrane Transport
        • Lesson 20: Energy and Cell Respiration
        • Lesson 21: Photosynthesis
      • Unit 3: Project
      • Unit 3: Exam Review
      • Unit 3: Exam
    • Unit 4 >
      • EIS >
        • 22: The Human Body
        • 23: The Nervous System
        • 24: The Endocrine System
        • 25: The Integumentary System
        • 26: The Musculoskeletal System
        • 27: The Cardiovascular System
        • 28: The Respiratory System
        • 29: The Digestive System
        • 30: The Excretory System
        • 31: The Immune System
      • Unit 4 Project
      • Unit 4 Exam Review
      • Unit 4 Exam
    • Units 5-6 Coming Soon
  • Donate

Immunity

Now that we’ve learned about all the microorganisms out in the world that could potentially hurt and kill you, let’s learn about the reasons that they don’t: your immune system. Your immune system consists of 3 main layers of defense. The first is passive: things like skin and stomach acid that just keep out and kill off bacteria without your body really having to do anything differently. It keeps out most stuff. For the stuff that gets through, we have a nonspecific cellular system that recognizes the general features of bacteria and kills them off. Finally, we have a super-specific, ultra-elite squad of B cells and T cells that take care of whatever sneaky little demons somehow manage to weasel their way past the first two layers. Let’s learn more about each of these.

The first line of defense

The first line of defense sometimes isn't classified as part of the immune system, but it is nonetheless very important to keeping diseases at bay. It prevents germs from getting into your body in the first place. Think of it like putting up fortifications around a city: sure you can have soldiers and spies and all sorts of weapons, but what will really keep you safe from the enemy is preventing them from getting into your city. If they can't do this, then they are completely harmless.

Here are a couple of fortifications our bodies put up against pathogens:

Your skin is an organ that covers the entire body and separates it from the outside world. Besides acting as a physical barrier to germs, its slight acidity helps in the immune response by killing off the bacteria that can't survive in an acidic environment. It also secretes an oil called sebum, which lubricates the skin. This provides an added layer of protection against invading pathogens. Also, sweat glands in your skin keep the surface too salty for many bacteria to survive, and also flush away germs. It's like your skin has a built in shower! Sort of. Please take regular showers.

Mucous membranes line anything on the inside of your body that touches the outside world. This includes your nose, mouth, throat, anus, and urinary tract. Like skin, it acts as a physical barrier to prevent germs from getting into the bloodstream or to your vital organs. It also secretes mucus, which traps germs. When you cough up mucus or blow your nose, you are getting rid of these trapped pathogens.

Some other things included in the first line of defense are:
  • Tears, which flush germs out of the eyes.
  • Urine, which flushes germs out of the urinary tract.
  • Stomach acid, which kills many germs on contact.
  • Nose hairs, which trap germs.
  • Coughing and sneezing, which expel germs from the respiratory tract. This is why expectorants (like Mucinex) are helpful when you have a cold--they get you coughing, which gets out the mucus and all of the germs trapped in it.
  • Some behaviors, such as leaning away from someone who sneezes.

The second line of defense

Though useful, having fortifications isn't always enough to prevent disease. So, let's say you get cut, or someone sneezes right in your face, or you nonchalantly touch a door handle that some sneaky dude with a cold touched mere moments ago. The enemy starts to invade. Now what?
Picture
We turn to our second line of defense. The second line of defense is also called the innate immune system because everyone has it from birth. It consists of leukocytes, or white blood cells, which circulate in the bloodstream. There are a few different types of leukocytes, but they all do essentially the same thing--phagocytize (eat) germs. Different chemical processes attract the phagocyte to the microbe. Once there, the phagocyte binds with the microbe and begins to engulf it. It then breaks down the microbe through a series of chemical processes, recycles the nutrients, and eliminates the waste. Eat your enemies!
Picture
Courtesy CrashCourse
Their mechanism of action may be a little different, but if the more advanced 3rd line of defense is like the super-elite squad of special agents and spies that know every in and out of one specific enemy looks like and know exactly how to target and kill that one guy, then the second line of defense is like an army. They know generally what the enemy looks like and they are very good at containing it. (Though, fortunately, modern warfare doesn’t really involve much cannibalism). 
Fever and inflammation are also part of the second line of defense. They are natural reactions to infection and are actually helpful. Fever creates an environment that is too warm for many germs to survive. Basically, you are frying them to death. Unfortunately, fever is also non-selective, so it can also harm your cells. For this reason, high or prolonged fevers can be extremely dangerous. 

Inflammation occurs when blood vessels in a certain area dilate so that more leukocytes can get to that area. When you are cut, nearby cells release a protein called histamine, which dilates blood vessels so that more leukocytes can get through. Blood vessel dilation also brings in more red blood cells, which is why inflamed areas are red, hot, and swollen. Meanwhile, different proteins called cytokines are released by dying cells as a sort of "SOS." They call leukocytes to destroy invading pathogens in the area. The leukocytes move out of the blood vessel and into the tissue, where they phagocytize invading germs. Here’s a video on inflammation:

the third line of defense

Say the enemy is a little too strong for soldiers alone--they've been sneaking around the city, outwitting even the strongest of soldiers and killing all of the citizens. It is time to employ the secret weapon. For the past two weeks, you have had spies tracking the enemy and learning their every move. Commence “Operation Antigen.”
Picture
Courtesy CrashCourse
The third line of defense is the most powerful line of defense because it is specific. It can take a few weeks to kick in because it spends so much time preparing the strongest attack possible. Cells called lymphocytes make up the third line of defense. These include B cells and T cells. B cells recognize bacteria and viruses that are floating around freely in your bloodstream. They release antibodies that prevent these pathogens from infecting your cells and also tag them for phagocytosis (kind of like tracking down the bad guy, alerting the soldiers, and letting them handle the dirty work). T cells recognize bad stuff in your cells once they’ve already gotten infected. They also recognize cancer cells. They then kill these cells before they can cause any more damage.
Picture
All lymphocytes have receptors that only recognize specific antigens. Antigens are special markings on microbes that make them recognizable. Each lymphocyte has a different type of receptor, so it will match up with a different microbe. Through mostly trial-and-error, each lymphocyte binds to the offending antigen to see which creates the best fit. The one with the best fit then replicates to form a specialized "army" of lymphocytes. This is called clonal selection. The clone army then goes to unleash its specialized antibody weapon—specifically targeted to a specific antigen—on offending viruses or bacteria, in the case of B cells, or directly kill infected (or cancerous) cells, in the case of T cells.

After the offenders are successfully destroyed, most of the lymphocytes disappear. The rest are converted to memory cells which will recognize the antigen more quickly if you get infected by the same thing again. "We will never forget!"

Summary

This video does a great job of summing up what I just described:
If you want a more complete story, this series does a great job of explaining the different levels of your immune response:
Immune System, Part 1
Immune System, Part 2
Immune System, Part 3
You should understand:
  • That the first line of defense provides passive defense against pathogens getting into the body in the first place, like the skin, which provides a barrier.
  • That the second line of defense involves non-specific cellular attacks that recognize the general features of bacteria and then do phagocytosis (eating) to destroy them. The second line of defense also includes fever, which tries to destroy invaders through heat, and inflammation, which is the swelling of blood vessels to get more phagocytotic cells to a damaged tissue.
  • That the third line of defense is super-specific and is made up of lymphocytes that specifically identify a single antigen and then, through clonal selection, expand to create a massive army of lymphocytes specifically designed to attack that antigen. 
  • That B cells attack viruses and bacteria before they get into cells, and T cells attack infected and cancerous cells.
  • That memory cells persist after an infection and attack a specific antigen again immediately the next time your body sees it, so that you don’t get sick from the exact two things twice.

Learning Activity

Picture
Lesson 31: The Immune System EIS

Content contributors: Emma Moulton and Emily Zhang
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