Immune System


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My AP Biology Thoughts

Unit 4 Cell Communication and Cell Cycle

Welcome to My AP Biology Thoughts podcast, my name is Jacqueline Sun and I am your host for episode 85 called Unit 4 Cell Communication and Cell Cycle: The Immune System. Today we will be discussing how your body defends itself against infection and stays healthy through the complex yet vital immune system.

Segment 1: Introduction to The Immune System

  • The immune system is responsible for fighting off germs and other harmful outside invaders from entering and wreaking havoc on your body
  • Different from all other systems in that it is not a specific cell, tissue, or organ but is rather composed of several different cells, tissues, and organs working in tandem.
  • There are many important components of the immune system that can be divided into two general categories: the innate (or the non specific defense system), and the adaptive (or the specific defense system).

Segment 2: More About The Immune System

Innate (Nonspecific defense system). As the name suggests, you are born with it and it is automatically activated in the face of threats.

  • Uses physical and chemical barriers, killer cells, and fevers to defend the body
  • Simple Physical Barrier is the first aspect of the innate immune system
  • The skin is the first line of defense. It covers nearly every part of your outside body and acts as a physical barrier and keeps out many harmful microorganisms. As long as the skin is mostly undamaged or unharmed, its defense will hold.
  • Mucous membranes also provide a physical barrier. They line the body cavities that connect the inside of your body to the outside, such as the respiratory and urinary system. The thick viscosity of the mucus aids in preventing the entry of harmful microorganisms.
  • Both of these physical barriers also provide chemical defense. For example, The skin and stomach can secrete acids to kill off invaders. Sweat, which is also secreted by the skin, has antimicrobial proteins to kill bacteria. Enzymes can be found in mucus that perform the same function.
  • So lets say your skin was scraped up. Now your body relies on the second line of internal innate defense.
  • Your body may instigate a fever, send out chemical signals calling for defense from body parts, or cause inflammation. These are just a few of the main tactics the body may use, there are many more.


  • Inflammatory response: A bodily process that uses chemicals to signal to the immune system defenders where the damage to the body is.
  • Utilizes redness, swelling, heat, and pain as signal markers
  • For example, In the event of injury, cells in the connective tissue will send out histamine molecules
  • Causes vasodilation, causing redness and heat, which increases the injured cells metabolic rate so they may repair themselves faster
  • Histamines and other inflammatory molecules will increase the permeability of blood vessels, causing nearby capillaries to release protein ridden fluids that cause swelling.
  • Symptoms like hat and swelling attract immune cells such as phagocytes and lymphocytes to start consuming the invading pathogens

What are the actual lines of defense coming to defend the body from intruders and pathogens?

  • Phagocytes are some of the first defense cells on the scene. Their name is directly translated to “to eat” and they indiscriminately consume invaders.
  • Neutrophils are the first type of phagocytes. THey are the most abundant types of white blood cells and after eating a pathogen will self destruct. They are released by injured skin cells in a process known as leukocytosis.
  • Macrophages are the bigger phagocytes and often the second line of defense. They are white-blood cells that have occupied tissues and organs. Some will freely move around and consume intruders, while others remain bound to the tissue or organ and defend against intruders. Unlike neutrophils, macrophages do not self-destruct after consuming a pathogen. They can consume and neutralize harmful invaders over and over again.
  • When the phagocytes defenders run into more pathogens than they can consume, they will signal the hypothalamus to raise body temperature, or cause a fever. This heat allows for cells increase their metabolism to heal themselves faster and also helps kill harmful cells.
  • Natural Killer Cells are another form of defense cell. They travel through the blood and lymph and seek out abnormal cells to kill. They are unique in that they may attack your own cells if they have become infected with viruses or have become cancerous.
  • If the natural killer cells detects a malevolent cell, it will introduce it to an enzyme that triggers cell death, or apoptosis.

However, sometimes your body is facing a more pervasive, dangerous intruder or illness. In this case, the innate defense system is not sufficient for defense, which is where the adaptive (or specific) immune system comes in.

Adaptive Immune system

  • Unlike the innate immune system, the adaptive immune system is not automatically activated and must be introduced to a specific pathogen and recognize it as as a threat before it will attack
  • You are not born with it, and it must be developed over time as your body comes into contact with harmful outsiders. This may occur randomly or be premeditated, which occurs through vaccination
  • Once it's been introduced to a threat, the adaptive defense will never forget it. This is one of the key differences between adaptive and innate defenses, for innate defense attack indiscriminately and do not recognize specific threats.
  • The adaptive system is also systemic, fighting throughout the whole body rather than one region.
  • Can be split into two defenses: humoral immunity and cellular defenses
  • Humoral Immunity: Works by dispatching antibodies, or special white blood cells that patrol the blood and lymph
  • They combat the bacteria and viruses in the spaces between cells
  • SO HOW does humoral immunity work exactly?
  • The adaptive immune system must first be able to recognize antigens, an invader such as a bacteria, virus, toxin, fungus, or diseased cell in your own body. Upon recognition, the immune system will flare up and take action.
  • B lymphocytes can be considered one of the first lines of defense for the humoral response. The B cell originates from and matures in the bone marrow and develops immunocompetence - ability to recognize and bind to a specific antigen, and self-tolerance - ability to not attack the body’s own cells.
  • When it has reached maturity, the B cell will have over 10,000 bound antibodies on its surface (every b cell has unique bounded antibodies)
  • When a B cell finally meets an antigen it has the unique antibodies for, it will recognize and bind to it. This activates large-scale replication of the cell, producing an army of cells with the same antibodies and with the same instructions of finding and fighting the same antigen.
  • Effector and memory cells are produced. Effector cells are the warriors who mass produce antibodies for a specific antigen at a rate of around 2000 antibodies per second.
  • Antibodies don’t do the killing themselves, but they use the strategy of neutralization, where they physically block binding sites on the antigens to prevent access to body tissues. They also signal the innate immune system, calling in phagocytes and special lymphocytes to kill the antigens
  • Some of the replicated B cells will become memory cells. Memory cells remain even after the antigen has been defeated, and they allow for a faster and stronger immune response in the future if the antigen is ever introduced again.

When the innate and humoral systems have failed and the cell themselves have been breached, cells must now fight within or among themselves to get rid of pathogens.

The key here is the T lymphocyte cell. T cells go after hijacked cells and cause inflammation, activate macrophages, and activate other T cells. T lymphocytes and other phagocytes in innate immune system engage in a specific process where after consuming the harfum pathogen, they will break the pathogen into small pieces and display them on protein grooves on surface of the cell.

These proteins are called major histocompatibility complexes, or MHCs. The cells that do this are known as professional antigen presenting cells, and they include macrophages and b cells. When the antigen presenting cells bind to the antigen fragments, which are now presented on their surface, they allow T cells to recognize the antigen fragments.

Several different types of T cells: two most important are the helper T cells and the cytotoxic cells

Helper Ts: Can’t actually kill pathogens but will activate other cells that can

When the Helper T cells bind to the MHCs that have antigens compatible with the T cell’s antibodies, it will be activated and replicate rapidly, sending out warning signals known as cytokines to alert the immune system of a problem. These cytokines will often activate cytotoxic cells.

Cytotoxic cells are the cells that do the actual killing of pathogens

They roam the blood and lymph, looking for infected cells to dispose of. It does this by triggering apoptosis in the infected cells through enzymes.

Segment 3: Connection to the Course

  • Ok, so why is the immune system so important? The immune system is your bodies primary defense system against any harmful bacteria, virus, or in general dangerous outside substances. It is active every second of the day, constantly recognizing and defending against intruders and keeping you healthy. The immune system uses a variety of cells, tissues, and organs to function, also employing different enzymes and proteins. Cell and chemical signalling are vital communication components of the immune system that help target and kill pathogens, whether this may be through consumption or apoptosis. Even though immune systems vary in complexity among different organisms, basic, innate immune responses still exist in every being that are geared towards the organism’s survival. Without a properly functioning immune system, any living thing, no matter how advanced or developed, will have a very short lifespan.

Thank you for listening to this episode of My AP Biology Thoughts. For more student-ran podcasts and digital content, make sure that you visit See you next time!

Music Credits:

  • WeVideo Licensed Music: MegaMusic

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