The body’s response

Lesson 3

How the body responds to infectious diseases through the immune system.

Learning outcomes & key terms

• Students will be able to define and explain the function and purpose of the immune system as the body’s response to infectious disease

• Students will compare and contrast three lines of defense or barriers that the body uses to keep pathogens at bay and categorise them as non-specific or specific

• Students will be able to deconstruct the third line of defense or the immune response using key scientific terminology including the role of lymphatic system

• Students will recognize the difference between active and passive immunity and where vaccinations fit into this

Immune system

The immune system is the body’s defense against infectious organisms and other invaders.

Immune response

The immune system attacks organisms and substances that invade body systems and cause disease in 3 distinct stages.


Pathogens are defined as microbes capable of causing host (human) damage. When host damage reaches a certain threshold, it can manifest itself as a disease

Innate immunity

Aka non-specific immunity. Generalised, responds to all kinds of immunity the same way. Its job is to keep the enemy from getting in

Adaptive immunity

Aka specific defence or acquired immunity

Does things differently to innate immunity: it has to be introduced to a specific pathogen and recognise it as a threat before it goes to action

Not born with this immune system. Can happen organically (touch a dirty door knob) or planned (vaccinations)

Once introduced, adaptive system never forgets it. It is systemic, fights throughout body

1st response 

The body’s mechanical, chemical and biological barriers. Surfaces of the skin (mechanical) physically prevent pathogens coming in, breathing passages, mouth, stomach (chemicals) – all mucous contains enzymes which trap and kill most pathogens with which the enzymes come in contact

Non pathogenic bacteria (biological) on skin and in GI tract take up space and resources to prevent other harmful microorganisms from surviving. Includes skin, mucous membranes, cerumen, tears, sweat and saliva, hairs and cilia

2nd response: Inflammatory response 

When body cells are damaged, they release chemicals that trigger the inflammatory response. Fluid and white blood cells actively move from blood vessels into nearby tissues

White blood cells called phagocytes then fight pathogens by engulfing them and breaking them down. Inflammatory response is the body’s early reaction against infection or tissue damage. Symptoms of inflammation: redness, pain and fever

3rd response: The immune response

If a pathogen manages to penetrate the first line, and survive the second line, a third line of specific response is then triggered – the immune response

The immune response is controlled by the immune system, in particular, the lymphatic system, the body’s disease-fighting system. Cells of immune system can distinguish between different kinds of pathogens

Immune system cells (lymphocytes) react to each different kind of pathogen with a defense targeted specifically at that pathogen

They allow the body to remember and recognize previous invaders and help the body destroy them

This last defense relies on the antigens and contains Helper T-Cells, B-Cells, antibodies, Memory B/T Cells, Killer T-Cells and Suppressor T-cells

Humoral response

Also called antibody-mediated response

Protective function of immunization is found in the humor or cell-free bodily fluids including secreted antibodies and proteins

B cells protect us from bacteria. Antigens are molecules on the cell, which our body recognizes as friend or foe

Cell-mediated response

An immune response that doesn’t involve antibodies but rather involves the activation of human inflammatory cells: phagocytes, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines (human chemicals) in response to a viral antigen

T cells are involved in fighting viral antigens

Humoral and cell-mediated responses can occur in both the innate and adaptive immune system


A type of white blood cell that engulfs and destroys a disease-causing antigen

Helper T cells

Attach themselves to an antigen on the surface of a macrophage. They tell other cells that there is an invading pathogen

B cells

Produce antibodies when they receive messages from Helper T cells


Produced from B cells, they attach themselves to the antigens to make it easier for the macrophage to engulf the invaders

Killer T Cells

When they receive a message from the Helper T cells, they are stimulated to release proteins that kill the invading pathogen

Memory B cells/T Cells

Created to remember the invading pathogen after it has been destroyed. They provide immunity against future invaders with the same specific antigen

Suppressor T cells

Signals the immune system to stop the response to the antigen by releasing a chemical messenger (cytokine)

Multi-cellular organisms rely on coordinated and interdependent internal systems to respond to changes in their environment (ACSSU175)


  • Describing how the requirements for life (for example oxygen, nutrients, water and removal of waste) are provided through the coordinated function of body systems such as the respiratory, circulatory, digestive, nervous and excretory systems
  • Explaining how body systems work together to maintain a functioning body using models, flow diagrams or simulations
  • Investigating the response of the body to changes as a result of the presence of micro-organisms

A survivor story

Prof Robert Booy talks with Gary Newton, a survivor of Polio – Polio is now vaccinated against and through immunisation is almost entirely eradicated from the earth

About the immune system

The immune system is the body’s defence against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks and neutralises organisms and substances that invade body systems and cause disease

The immune system has two avenues to fight disease:

  • non-specific immunity, and
  • specific immunity


Non-Specific Immunity

Also known as innate immunity. This is the first line of defence once a pathogen threatens to enter the body. An example is the macrophage, which gobbles up pathogens!


  • All animals have non-specific immunity
  • White blood cells do the majority of the work for non-specific immunity

White Blood Cells

These are special cells because they can:

  • ingest pathogens and destroy them,
  • produce chemicals to destroy pathogens and they can neutralise the chemicals released by pathogens that make us sick

If a pathogen does enter the body, such as through a cut in the skin, the innate immune system will send cells to the area

The White Blood Cells will help sterilise the area, ingest and destroy any pathogens and start to heal and clot the wound to prevent any further injuries or more pathogens entering

When the innate system is at work, quite often inflammation and fever follow. Inflammation enables blood carrying immune cells to flow to the infected area more easily

White blood cells come in a variety of types: phagocytes, macrophages, lymphocytes, and neutrophils

Each type of white blood cell has a purpose to achieve a quick response in the case of pathogen infection


Specific Immunity

Specific immunity is sometimes referred to as adaptive immunity. It is slower to respond but it can learn from diseases and form a memory. Once it has seen a disease once and formed a memory, its response is rapid and effective

It is because of specific immunity that vaccinations work

  • Only vertebrates have specific immunity
  • Lymphocytes are involved in specific immunity

The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. One of the important cells involved are white blood cells, also called leukocytes, which come in two basic types that combine to seek out and destroy disease-causing organisms or substances

There are two basic types of leukocytes:

  1. phagocytes, cells that chew up invading organisms (non-specific or innate immunity) and
  2. lymphocytes, cells that allow the body to remember and recognize previous invaders and help the body destroy them

A number of different cells are considered phagocytes. The most common type is the neutrophil, which primarily fights bacteria. If doctors are worried about a bacterial infection, they might order a blood test to see if a patient has an increased number of neutrophils triggered by the infection. Other types of phagocytes have their own jobs to make sure that the body responds appropriately to a specific type of invader

The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start out in the bone marrow and either develop there and mature into B cells, or they leave for the thymus gland, where they mature into T cells

B lymphocytes and T lymphocytes have separate functions: B lymphocytes are like the body’s military intelligence system, seeking out their targets and sending defences to lock onto them

T cells are like the soldiers, destroying the invaders that the intelligence system has identified

Leukocytes are produced or stored in many locations in the body, including the thymus, spleen, and bone marrow. For this reason, those places called lymphoid organs. There are also clumps of lymphoid tissue throughout the body, primarily as lymph nodes, that house the leukocytes

The leukocytes circulate through the body between the organs and nodes through both lymphatic vessels and blood vessels. In this way, the immune system works in a coordinated manner to monitor the body for germs or substances that might cause problems

Putting it all together: 3 lines of defence

Putting all the pieces together. Our first line of defence is barriers like skin and mucus

If pathogens get past that then the non-specific (innate and immediate) immune system is activated

If the specific immune system has seen the pathogen before it can respond more quickly and destroy the pathogen but if the germ is new to the body’s immunity, it takes a few days or a week (depending on the pathogen) to react and defend the body

Here’s how it works

When antigens (foreign substances, usually on the outside of a germ, that helps invade the body) are detected, several types of cells work together to recognize them and respond. These cells trigger the B lymphocytes to produce specific antibodies, which are specialised proteins that lock onto specific antigens

Once produced, these specific antibodies stay in a person’s body, as do mature lymphocytes with memory (called plasma cells, that can convert to rapidly make a specific antibody). If his or her immune system encounters that antigen again, the antibodies are already there to do their job, and the plasma cells switch on quickly. So if someone gets sick with a certain disease, like chickenpox, that person usually won’t get sick from it again

This is also how immunisations prevent certain diseases. An immunisation introduces the body to a particular foreign antigen in a way that doesn’t make someone sick, but does allow the body to produce antibodies (and memory cells) that will then protect the person from future attack by the germ or substance that produces that particular disease

Although antibodies can recognise an antigen and lock onto it, they are not always capable of destroying it without help. That’s the job of the T cells, which are part of the system that destroys antigens that have been tagged by antibodies or cells that have been infected or somehow changed. (Some T cells are actually called “killer cells.”) T cells also are involved in helping signal other cells (like phagocytes) to do their jobs

Antibodies also can neutralise toxins (poisonous or damaging substances) produced by different organisms. Lastly, antibodies can activate a group of proteins called complement that are also part of the immune system

Complement assists in killing bacteria, viruses, or infected cells

All of these specialised cells and parts of the immune system offer the body protection against disease. This protection is called immunity

What about Coronavirus (COVID-19) vs the Flu?

What are the symptoms and effects of COVID-19 on humans and what’s the difference between Flu and Covid-19?

Watch this video on how the body responds to Coronavirus with Prof Robert Booy and the Academy of Sciences.

Class activity

Part 1


1. What are the 4 main categories of pathogens?

2. What is an example of a disease caused by each?

3. How does it enter the body?


  • Bacteria (Strep throat, E. coli, staph infections, tuberculosis, tetanus, syphilis)
  • Virus (cold, flu, chicken pox, HIV, cold sore, measles)
  • Fungi (athlete’s foot, Ringworm, tinea, candidiasis)
  • Parasites (diarrhea, malaria, giardiasis)


Teacher led discussion about Immune system.

  • What stops a pathogen killing us/making us sick straight away?
  • Where is the immune system located?
  • How does it work?


Part 2

1. Build your definitions

Students are to write down each of the following terms, do research on the content in this lesson and online and match a definition to each key term


  • Immune System
  • Immune Response
  • Pathogen
  • First line of defense
  • Second line of defense / Inflammation response
  • Third line of defense / Immune Response
  • Non-specific response
  • Specific response
  • Antigen
  • Macrophages
  • B cells
  • Antibodies
  • Killer T cells
  • Helper T cells
  • Memory B and T cells

Part 3

1. Build your battle scene

There are some great analogies for the immune system like robbing a bank or protecting a castle. Using the analogy of protecting a castle (or choose your own), explain how the immune system works. (see examples of analogies in Teacher Resources)



Create an image of a castle battle scene that students refer to or have them create their own

Label the battle scene using the castle example and definitions of the different players and parts of the immune response

Students work in small groups to label components of 1st, 2nd and 3rd line of defence

At the end, groups of students describe what is happening in the scene using analogy and then scientific terms they represent advanced explanation of immune system. Your immune System: Natural Born Killer. Crash Course Biology #32

More basic explanation: animation of how the immune system works (direct link: ) – during Emmanuelle Charpentier’s studies of Streptococcus pyogenes, one of the bacteria that cause the most harm to humanity, she discovered a previously unknown molecule, tracrRNA. Her work showed that tracrRNA is part of bacteria’s ancient immune system, CRISPR/Cas, that disarms viruses by cleaving their DNA.

Short videos including immune immune system structure and function – overview, non-specific and specific response. Unit – Barriers to Pathogens.

If pathogens get through the body’s first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. For a cartoon introduction to the immune system, watch this video: Unit plan on acting out the immune response. Great ideas for extension. Unit plan studying the body’s defense mechanisms YouTube video: Your Immune System: Natural Born Killer by Crash Course Biology (advanced)

Analogies of Immune System:

Robbing a bank. Protecting a castle Infection, detection, protection. Series of activities and games focusing on the immune system. These are geared at younger learners but provide good ideas for adapted technology creation. more aligned with lesson 2, this is a great idea for an interactive game though.

Which type of barrier to pathogens is in your opinion the most effective? Explain why.

Class extension activity: In the class activity “Acting Out the Immune Response” at the URL below, students will act out a nonspecific and then a specific response to an invading pathogen. Each student will have an assigned role in the activity, and props will be used.


The body has an orderly way of fighting disease through an impressive 3 lines of defense

In the next lesson we will look in further detail at how we develop immunity and the role of vaccines in preventing disease in the first place


1) Non-specific immunity is
a) The first line of defense against a pathogen
b) Antibodies
c) Only present in some animals
d) All of the above


2) White blood cells
a) Are the cells that carry oxygen to the body
b) Are the cells that cause tissue growth
c) Ingest pathogens to destroy them
d) Have only one type


3) Specific immunity is
a) Initially slower to respond than non-specific immunity
b) Initially faster to respond than non-specific immunity
c) Is only found in invertebrates
d) Involves red blood cells


4) Immunisations prevent diseases by
a) Killing the pathogen by phagocytosis
b) Teaching the body to produce antibodies the same way the pathogen would, but without making someone sick
c) Releasing complement to kill the pathogen
d) Mimicking the pathogen