🛡️ Immunity and Immune System¶
Exam Importance: ⭐⭐⭐⭐⭐ (Very High)
This topic appears in 90% of previous final exams. Focus especially on:
- Types of immunity (Innate vs Adaptive)
- Antibodies and Antigens
- Vaccination and how vaccines work
- Autoimmune diseases
What is Immune System?¶
In simple terms, it is the study of our body's defense mechanisms against infections and disease.
The immune system is a complex network of organs, cells and proteins that defends the body against infection, whilst protecting the body's own cells.
The immune system keeps a record of every antigen (pathogen/germ/foreign/harmful substances) it has ever defeated so it can recognize and destroy the antigen quickly if it enters the body again.
Why don't we get sick often?
Even though we are constantly bombarded by microorganisms, we rarely become sick. This is because of our immune system!
Components of Immune System¶
The immune system is made up of special organs, cells and chemicals that fight infections (microbes) and Cancers.
Main Parts:¶
| Component | Function |
|---|---|
| Thymus | T-cell maturation |
| Spleen | Filters blood, stores white blood cells |
| Lymph Nodes | Filter lymph fluid, trap microbes |
| Bone Marrow | Produces blood cells including immune cells |
| Adenoids & Tonsils | First line defense in throat |
| Complement System | Proteins that help destroy pathogens |
| Lymphatic System | Network of vessels carrying lymph |
| White Blood Cells | Active fighters against infection |
| Antibodies | Proteins that neutralize pathogens |
Types of Immune System¶
1. Innate (Non-specific) Immune System¶
Characteristics:
- ✅ Innate, not developed after exposition to infection
- ✅ Uniform response, prompt, no immunological memory
- ✅ First line of defense
Components:
- Mechanical barriers (mucosa, skin)
- Phagocytic cells (microphages, macrophages)
- Acute phase proteins (CRP)
- Complement system
Key Point
It is the immunity that is present at birth and lasts a person's entire life. Innate immunity is the first response of the body's immune system to harmful foreign substances.
2. Adaptive (Specific) Immune System¶
Characteristics:
- ✅ Adaptability, developed after exposition to infection
- ✅ The immune response is not inherited
- ✅ Has immunological memory
Components:
- B cells and T cells
- Immunoglobulins
- Antibodies
Remember
We are born with our innate immune system almost fully developed. The adaptive immune system develops later and grows as we age and conquer new infections over time.
Human Immune System Overview¶
Innate vs Adaptive Immunity Comparison¶
Other Body Defenses Against Microbes¶
| Defense | Mechanism |
|---|---|
| Skin | Waterproof barrier that secretes oil with bacteria-killing properties |
| Lungs | Mucous traps foreign particles, cilia wave mucous upwards |
| Digestive Tract | Mucous lining contains antibodies, stomach acid kills microbes |
| Body Fluids | Skin oil, saliva, tears contain anti-bacterial enzymes |
| Urinary Tract | Constant flushing helps remove pathogens |
Lymphatic System¶
The lymphatic system is a network of delicate tubes throughout the body that:
- ✅ Manages fluid levels in the body
- ✅ Reacts to bacteria
- ✅ Deals with cancer cells
- ✅ Deals with cell products that would cause disease
- ✅ Absorbs fats from the intestine
Components:
- Lymph nodes - trap microbes
- Lymph vessels - tubes carrying lymph fluid
- White blood cells (lymphocytes)
The Immune System and Microbial Infection¶
Recording Function: The immune system keeps a record of every microbe it has ever defended, in types of white blood cells (B- and T-lymphocytes) known as memory cells.
Exception
Some infections, like the flu and common cold, have many different strains, which is why we can catch them multiple times.
White Blood Cells¶
Key Facts:
- Formation: Bone marrow, part of lymphatic system
- Process: Move through blood and tissue looking for foreign invaders
- Types: Lymphocytes (B-cells, T-cells, Natural Killer cells) and others
Main Functions of White Blood Cells¶
Antibodies (Immunoglobulin)¶
Definition: A protein made by plasma cells (a type of white blood cell) in response to an antigen.
Key Points:
- Each antibody can bind to only one specific antigen
- Purpose: Help destroy the antigen
- Some destroy antigens directly, others help white blood cells destroy them
Types of Antibodies¶
| Type | Description |
|---|---|
| IgG | Most abundant (70-75% of total), found in blood plasma |
| IgM | First antibody produced in immune response |
| IgA | Found in mucous membranes |
| IgD | Functions in B cell activation |
| IgE | Involved in allergic reactions |
Monoclonal vs Polyclonal Antibodies¶
| Type | Description |
|---|---|
| Monoclonal | Homogeneous preparations, single type of antigen binding site, produced by single B cell clone |
| Polyclonal | Complex mixtures from immunized animals, produced by many different B cell clones |
Antigens¶
Definition: Any substance that causes the body to make an immune response against it.
Examples:
- Toxins, chemicals, bacteria, viruses
- Other substances from outside the body
- Cancer cells and body tissues also have antigens
The Complement System¶
The complement system consists of approximately 40 proteins that:
- Circulate in blood and tissue fluids
- Participate in immune system function
- Most are normally inactive but activate in response to microorganisms
Functions:
- ✅ Augmenting antibody responses and immunologic memory
- ✅ Lysing (breaking down) foreign cells
- ✅ Clearing immune complexes and apoptotic cells
- ✅ Stimulating chemotaxis
- ✅ Triggering mast cell degranulation
Bridge Function
The complement system bridges innate immunity and acquired immunity.
Natural Killer (NK) Cells¶
Key Functions:
- Attack body's own cells infected by viruses
- Attack potential cancer cells (often before tumors form)
- Bind to cells using an antibody "bridge"
- Kill by secreting perforin (makes holes in cell membrane)
Activation Mechanisms:
- Cancer cells and infected cells release activating signals
- NK cells release perforin and granzymes
- NK cells secrete cytokines (IFNγ, TNFα) to enhance immune response
Mast Cells¶
Location: Connective tissues, under skin, near blood vessels, nerves, lungs, intestines
Contents:
- Histamine
- Heparin
- Cytokines
- Growth factors
Effects of Histamine Release (Inflammation):
| Symptom | Cause |
|---|---|
| Redness | Blood vessel dilation |
| Swelling | Tissue fluid entering area |
| Pain | Nerve stimulation |
| Heat | Increased blood flow |
Vaccination and Immune System¶
Everyday we face many pathogens:
- 🦠 Bacteria
- 🪱 Parasites
- 🍄 Fungus
- 🧫 Viruses
But your immune system helps to protect you!
Common Disorders of the Immune System¶
Overactive/Hyperactive Immune System¶
Allergic Diseases:
- Immune system makes overly strong response to allergens
- Examples: food allergies, medication allergies, hay fever, asthma, hives, dermatitis, eczema
Autoimmune Diseases:
- Immune system attacks normal body components
- Examples: multiple sclerosis, autoimmune thyroid disease, type 1 diabetes, lupus, rheumatoid arthritis
Underactive Immune System (Immunodeficiency)¶
Causes:
- Inherited (e.g., CVID, SCID)
- Medical treatment (corticosteroids, chemotherapy)
- Other diseases (HIV/AIDS, certain cancers)
Organ Transplant
People who have had organ transplants need immunosuppression treatment to prevent the body from attacking the transplanted organ.
Cellular Response / Immunity¶
Process:
- Pathogen invades body
- Macrophages engulf and display antigens
- This activates helper T (Th) cells
- Helper T cells activate B cells, cytotoxic T cells, NK cells, and macrophages
- Cytotoxic T cells release cytokines
- B cells produce antibodies
How Does the Immune System Fight?¶
Innate vs Adaptive Response¶
| Innate Response | Adaptive Response |
|---|---|
| Myeloid cells | Lymphoid cells |
| Identifies "My cells" from "Not my cells" | Makes specific antibodies |
| Swallows pathogens whole | Antibodies signal for destruction |
How Vaccines Work¶
Physical Barriers:
- Skin, mucus, cilia prevent pathogens from entering
Vaccine Mechanism:
- Vaccines contain weakened or inactive parts of organisms (antigens)
- Newer vaccines contain blueprints for producing antigens
- Won't cause disease but triggers immune response
- Some require multiple doses for long-lived antibodies and memory cells
How Vaccines Help¶
Benefits:
- Body is trained to fight specific disease-causing organisms
- Builds memory of the pathogen
- Rapidly fights pathogen if exposed in future
Herd Immunity¶
Herd Immunity
When a lot of people in a community are vaccinated, the pathogen has a hard time circulating because most people it encounters are immune.
Importance:
- Protects people who cannot be vaccinated (health conditions, allergies)
- No single vaccine provides 100% protection
- Community vaccination provides substantial protection
Remember: Vaccinating not only protects yourself but also protects those in the community who are unable to be vaccinated!
Vaccine Designs¶
| Type | Description |
|---|---|
| Whole-inactivated | Inactivated virus/bacteria |
| Synthetic peptide | Laboratory-made piece of protein |
| Recombinant viral vector | Another virus carries pieces of pathogen |
| mRNA | mRNA carries pieces of pathogen |
| DNA | DNA carries pieces of pathogen |
| Live-attenuated | Weakened pathogen |
| Virus-like particles | Same shape, modified insides |
Vaccines: Benefits and Risks¶
Benefits ✅¶
- Provide protection from pathogens
- Control and eradication of diseases (polio, smallpox)
- Stop outbreaks and global epidemics
Risks/Challenges ⚠️¶
- No medicine is 100% effective
- Side effects (swelling, fevers)
- Not all pathogens have vaccines (e.g., malaria)
Immunisation (HALO Framework)¶
Immunisation needs are decided by HALO:
| Factor | Description |
|---|---|
| Health | Health conditions may require additional vaccines |
| Age | Different ages need different vaccines |
| Lifestyle | Travel, activities may require extra vaccines |
| Occupation | Work exposure may need additional immunisation |
Summary: Immune Responses¶
📝 Exam Practice Questions¶
!!! question "Frequently Asked Questions" 1. Name some autoimmune diseases and their impact on health 2. Name the types of vaccines used against COVID-19 3. Name the components of the immune system 4. Discuss the relationship of vaccination with primary and secondary response 5. Differentiate between innate and adaptive immunity 6. What are antibodies? List the types 7. Explain herd immunity