CIE AS/A Level Biology -11.1 The immune system- Study Notes- New Syllabus
CIE AS/A Level Biology -11.1 The immune system- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -11.1 The immune system- Study Notes- New Syllabus
Key Concepts:
- describe the mode of action of phagocytes (macrophages and neutrophils)
- explain what is meant by an antigen (see 4.1.3) and state the difference between self antigens and non-self antigens
- describe the sequence of events that occurs during a primary immune response with reference to the roles of:
• macrophages
• B-lymphocytes, including plasma cells
• T-lymphocytes, limited to T-helper cells and T-killer cells - explain the role of memory cells in the secondary immune response and in long-term immunity
Mode of Action of Phagocytes (Macrophages and Neutrophils)
🌱 Overview
Phagocytes are white blood cells that engulf and destroy pathogens. Two main types: Macrophages (large, long-lived) and Neutrophils (smaller, short-lived). They are a key part of the innate immune system.
🔬 Steps in Phagocytosis
- Chemotaxis and Recognition: Phagocytes move toward chemical signals released by pathogens or damaged tissues. Receptors on phagocytes recognize antigens on pathogen surfaces.
- Engulfment: The phagocyte extends pseudopodia around the pathogen. Pathogen is enclosed within a vesicle called a phagosome.
- Formation of Phagolysosome: Phagosome fuses with a lysosome → forms phagolysosome. Lysosomal enzymes and reactive oxygen species digest and kill the pathogen.
- Exocytosis / Antigen Presentation: Waste material is expelled via exocytosis. Macrophages can also present antigen fragments on their surface to activate specific immunity (T cells).
📊 Comparison: Macrophages vs Neutrophils
| Feature | Macrophages | Neutrophils |
|---|---|---|
| Size | Large | Smaller |
| Lifespan | Long-lived | Short-lived |
| Function | Engulf many pathogens; antigen presentation | Engulf fewer pathogens; rapid response |
| Location | Tissue and blood | Blood |
| Activity | Slower, sustained | Fast, first line of defense |
– Phagocytes detect, engulf, and destroy pathogens.
– Macrophages also activate the adaptive immune system by presenting antigens.
– Neutrophils act quickly at infection sites but die soon after, forming pus.
– Phagocytosis is a non-specific defense mechanism, effective against many pathogens.
Antigens: Definition and Types
🌱 What is an Antigen?
Antigen: Any molecule or part of a molecule that can trigger an immune response.
Usually found on the surface of cells, pathogens, or foreign particles.
The immune system recognizes antigens and may produce antibodies against them.
🔬 Self Antigens vs Non-Self Antigens
| Type | Definition | Example |
|---|---|---|
| Self Antigens | Molecules naturally present on body’s own cells; normally not attacked by the immune system | MHC proteins on human cells |
| Non-Self Antigens | Molecules foreign to the body that trigger an immune response | Proteins on bacteria, viruses, pollen |
– Self antigens → help immune system recognize own cells → prevents autoimmunity.
– Non-self antigens → activate immune response to defend against infection.
– Recognition of self vs non-self is critical for immune system function.
Primary Immune Response
🌱 Overview
The primary immune response occurs when the body is exposed to a pathogen for the first time.
Involves innate and adaptive immune cells: macrophages, B-lymphocytes, and T-lymphocytes.
🔬 Sequence of Events
- Pathogen Recognition by Macrophages
- Macrophages engulf the pathogen via phagocytosis.
- Digest the pathogen and present antigen fragments on their surface (antigen presentation).
- Release chemical signals (cytokines) to activate other immune cells.
- Activation of T-helper (Th) Cells
- T-helper cells recognize antigen-MHC complexes on macrophages.
- T-helper cells release cytokines to:
- Stimulate B-lymphocytes
- Activate T-killer cells
- Stimulate B-lymphocytes
- Activation of B-lymphocytes
- B-cells with receptors specific to the antigen bind the pathogen.
- With signals from T-helper cells, B-cells proliferate and differentiate into:
- Plasma cells → produce antibodies specific to the antigen
- Memory B-cells → remain in circulation for faster response if pathogen returns
- Activation of T-killer (Cytotoxic) Cells
- T-killer cells recognize infected cells displaying antigen fragments.
- Destroy infected cells by inducing lysis, preventing pathogen replication.
📊 Summary Table: Roles of Cells in Primary Response
| Cell Type | Role in Primary Immune Response |
|---|---|
| Macrophages | Phagocytose pathogens, present antigens, release cytokines |
| T-helper (Th) cells | Recognize antigens, release cytokines to activate B & T-killer cells |
| B-lymphocytes | Differentiate into plasma cells → produce antibodies; form memory cells |
| T-killer (Tc) cells | Kill infected cells displaying antigen fragments |
– Primary response is slower (several days) because cells are encountering the antigen for the first time.
– Antibodies are produced by plasma cells to neutralize pathogens.
– Memory cells ensure a faster, stronger secondary response upon future exposure.
Memory Cells and the Secondary Immune Response
🌱 Overview
Memory cells are long-lived B-lymphocytes and T-lymphocytes formed during the primary immune response.
They are essential for rapid and effective responses if the same pathogen is encountered again.
🔬 Role in Secondary Immune Response
| Feature | Function |
|---|---|
| Rapid activation | Memory B and T cells recognize the antigen immediately upon re-exposure |
| Faster antibody production | Plasma cells derived from memory B-cells produce large quantities of antibodies quickly |
| More effective response | T-killer cells respond faster to destroy infected cells |
| Higher antibody affinity | Antibodies produced are often more specific and effective than during primary response |
Key Point: Secondary response is faster, stronger, and longer-lasting than the primary response.
🔬 Role in Long-Term Immunity
- Memory cells persist for years or even a lifetime, providing immunity without repeated infection.
- Basis for vaccination: exposure to an antigen in a vaccine forms memory cells without causing disease, protecting against future infections.
– Memory cells allow the immune system to “remember” pathogens.
– They ensure rapid, high-level protection during secondary exposure.
– Long-term immunity depends on persistence and effectiveness of memory B and T cells.