11.1.110-11 · Infectious Disease and Immunity
Phagocytes — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 11 parts Lesson 11 of 21 1 quiz
Lesson summary
11.1.1 Phagocytes — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the lines of defence, the types of phagocyte and the process of phagocytosis.First line — it keeps pathogens out.Intact skin with a keratinised outer layer and sebum.Mucus and cilia in the ai…
- Second line
- Third line
- Innate defences
- Inflammation
- Natural killer (NK) cells
- Complement proteins
Section 1 of 11Lines of defence
1First line
- First line it keeps pathogens out.
- Intact skin with a keratinised outer layer and sebum.
- Mucus and cilia in the airways.
- Stomach acid.
- Lysozyme in tears and saliva.
- Competing harmless gut bacteria.
2Second line
- Second line non-specific defences act on any pathogen that gets in: inflammation, fever and phagocytosis.
3Third line
- Third line the specific immune response by lymphocytes recognises particular antigens.
4Innate defences
- Innate defences the first two lines are present from birth and respond the same way to every pathogen.
Section 2 of 11Inflammation, fever and other defences
1Inflammation
- Inflammation damaged cells release chemicals such as histamine, so local blood vessels dilate and become leaky.
- Plasma and phagocytes move into the tissue.
- This produces redness, heat, swelling and pain.
2Fever
- Fever the hypothalamus raises body temperature in response to chemicals from pathogens and phagocytes.
- A higher temperature slows many bacteria and speeds some immune processes.
- A very high fever can damage the body's own enzymes.
3Natural killer (NK) cells
- Natural killer (NK) cells they destroy virus-infected and abnormal cells without needing a specific antigen.
4Complement proteins
- Complement proteins they punch holes in bacterial membranes and mark them for phagocytes.
Section 3 of 11Types of phagocyte
| Feature | Neutrophil | Macrophage |
|---|---|---|
| Origin | Made in bone marrow and stored there | Develop from monocytes made in bone marrow |
| Nucleus | Multi-lobed (usually 3 to 5 lobes) | Large, single, bean-shaped or rounded |
| Where found | Blood; move into infected tissue | Tissues, lungs, liver, spleen, kidney, lymph nodes |
| Life span | Short, hours to days | Long-lived |
| Role | First on the scene; engulf and kill pathogens and release enzymes | Engulf pathogens and present antigens to T cells |
Types of phagocyte
Neutrophil numbers
- Neutrophil numbers they are about 60 to 70 per cent of white cells and are released in large numbers during infection.
Pus
- Pus it consists largely of dead neutrophils and cell debris.
Common features
- Common features both phagocytose and both come from bone marrow stem cells.
Section 4 of 11Phagocytosis
Phagocytosis
Chemotaxis
- Chemotaxis the phagocyte detects chemicals released by pathogens or damaged cells and moves towards them.
- (chemoattractants, such as histamine)
Recognition and attachment
- Recognition and attachment receptors on the phagocyte surface bind to antigens on the pathogen.
Opsonisation by antibodies makes this more efficient.
Engulfment
- Engulfment the cell surface membrane extends around the pathogen and encloses it in a vacuole called a phagosome (endocytosis).
Fusion with lysosomes
- Fusion with lysosomes a phagolysosome forms and releases lysozyme and other hydrolytic enzymes.
Digestion
- Digestion the enzymes digest the pathogen, useful products are absorbed and waste is expelled.
Antigen presentation (macrophages only)
- Antigen presentation (macrophages only) fragments of antigen are displayed on the membrane to activate T lymphocytes.
Section 5 of 11Antigen presentation
1Incomplete digestion
- Incomplete digestion macrophages do not destroy pathogens completely.
2Antigen-presenting cell
- Antigen-presenting cell a macrophage that displays antigens on its surface.
3Recognition
- Recognition lymphocytes then recognise the displayed antigens.
4Initiating the specific response
- Initiating the specific response only macrophages become antigen-presenting cells.
Section 6 of 11Identifying white cells
Neutrophils
- Neutrophils multi-lobed nucleus and granular cytoplasm.
Lymphocytes
- Lymphocytes large round nucleus filling most of the cell with a thin rim of cytoplasm.
Monocytes
- Monocytes kidney-shaped nucleus and larger size.
Red blood cells
- Red blood cells smaller, more numerous and without a nucleus.
Section 7 of 11Why phagocytosis matters
Speed
- Speed it is rapid protection while the specific response develops.
Information
- Information it supplies antigen information that activates the specific response.
Limits
- Limits it is non-specific and gives no memory.
Terms
- Terms the enzymes released from lysosomes digest the pathogen, not the vacuole itself.
Section 8 of 11More from the specification
Clonal selection
- Selection and proliferation of lymphocytes with matching receptors.
Mitosis
- Cell division producing genetically identical cells.
Primary response
- The slow response on first exposure to an antigen.
Section 9 of 11More on this topic
- Fusion
- a lysosome fuses with the vacuole, forming the phagolysosome.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Phagocyte | A white blood cell that ingests and digests pathogens. |
| Chemotaxis | Movement in response to chemicals. |
| Phagosome | The vacuole enclosing an engulfed pathogen. |
| Phagolysosome | A phagosome fused with a lysosome. |
| Antigen-presenting cell | A cell that displays pathogen antigens on its surface. |
| Opsonisation | Coating of a pathogen with antibodies to help phagocytes ingest it. |
Section 11 of 11Exam technique
1
Saying neutrophils present antigens — only macrophages do.
2
Saying the vacuole digests the pathogen — enzymes from lysosomes do.
3
Saying phagocytosis is specific — it is non-specific.
4
Saying phagocytes give immunological memory — they do not.
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