The shape of the lesson
By the end of the lesson, learners can
- Engulf first, ask later: Press Start and watch the neutrophil hunt down the bacteria one by one.
- Self and non-self: Click each cell in the scene, or use the chips.
- Where phagocytes fit: The immune response has two parts that work together.
- Chemotaxis, attach, engulf into a phagosome, lysosomes fuse, hydrolyse.
- Neutrophils: many, short-lived, first to arrive.
- Macrophages: long-lived, present antigens to lymphocytes.
- Antigen = molecule that triggers a response; self tolerated, non-self attacked.
One picture every idea returns to
Phagocytes are white blood cells made in the bone marrow that engulf and digest pathogens. Neutrophils are short-lived, very numerous and the first to arrive at an infection; macrophages are larger, longer-lived cells that develop from monocytes in the tissues and, after digesting a pathogen, present its antigens to lymphocytes. Phagocytosis is non-specific: it is attracted by chemicals (chemotaxis), the pathogen is engulfed into a phagosome, lysosomes fuse with it and hydrolytic enzymes digest it. An antigen is a molecule, usually a protein or glycoprotein on a cell surface, that stimulates an immune response. Self antigens are the body's own and are tolerated; non-self antigens are foreign and trigger a response.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, The first line of cellular defence.. Within minutes of a cut, white blood cells are crawling towards the bacteria that got in. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Engulf first, ask later | Press Start and watch the neutrophil hunt down the bacteria one by one. Project tab 1 of the lab (Immunity lab, phagocytes) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Self and non-self | Click each cell in the scene, or use the chips. Project tab 2 of the lab (Immunity lab, self and non-self antigens) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Where phagocytes fit | The immune response has two parts that work together. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Where phagocytes fit, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Phagocytosis is non-specific: the same neutrophil will engulf any pathogen it recognises as non-self. | Do not say a phagocyte has receptors for one specific antigen; that is true of lymphocytes, not phagocytes. |
| An antigen is not a pathogen. | A pathogen carries many different antigens, and a single antigen is the molecule that a lymphocyte receptor or an antibody binds to. |
Phagocytes and antigens: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how a neutrophil destroys a bacterium.
Compare neutrophils and macrophages.
Explain what is meant by an antigen.
A patient receives a kidney from an unrelated donor. Explain why the patient's immune system may reject the kidney.
Suggest why the number of neutrophils in the blood rises sharply during a bacterial infection.
Explain why phagocytosis is described as a non-specific response, while the response of lymphocytes is specific.
Answer key and marking notes
- Chemotaxis: the neutrophil moves towards chemicals released by the bacterium or damaged cells.
- Receptors on the neutrophil bind to non-self antigens on the bacterium.
- The cell surface membrane extends around the bacterium and engulfs it into a phagosome (vacuole) by endocytosis.
- Lysosomes fuse with the phagosome and release hydrolytic enzymes (lysozyme) that digest the bacterium.
- Similarities: both are phagocytes made in the bone marrow that engulf pathogens and digest them with lysosomal enzymes.
- Neutrophils have a lobed nucleus, are short-lived and arrive at an infection first in large numbers.
- Macrophages are larger and long-lived, develop from monocytes in tissues, and present antigens to lymphocytes.
- A molecule (usually a protein or glycoprotein, often on a cell surface) that stimulates an immune response.
- It is recognised by complementary receptors on lymphocytes and leads to the production of specific antibodies.
- The donor's cells carry surface antigens (glycoproteins) that differ from the patient's own.
- These are recognised as non-self antigens.
- Lymphocytes with complementary receptors are activated, and T-killer cells and antibodies attack the kidney cells.
- Neutrophils are short-lived and many die after engulfing bacteria.
- The bone marrow makes more to replace them and to deal with the large number of bacteria, so the count rises.
- A phagocyte engulfs any material it recognises as non-self, whatever the pathogen.
- Each lymphocyte has receptors of one shape, complementary to only one antigen.
- So only the lymphocytes that match a particular antigen respond to it.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement about neutrophils is correct?
- Athey are short-lived phagocytes that arrive in large numbers early in an infection
- Bthey are long-lived phagocytes that present antigens to T-helper cells
- Cthey are lymphocytes that secrete antibodies against one antigen
- Dthey are phagocytes that only engulf pathogens already coated with antibodies
Which organelle supplies the enzymes that digest a bacterium inside a phagocyte?
- Alysosome
- Brough endoplasmic reticulum
- Cribosome
- Dmitochondrion
The diagram shows three white blood cells, P, Q and R. Which row identifies them correctly?
- AP neutrophil, Q macrophage, R lymphocyte
- BP macrophage, Q neutrophil, R lymphocyte
- CP lymphocyte, Q macrophage, R neutrophil
- DP neutrophil, Q lymphocyte, R macrophage
Which molecule is most likely to act as an antigen on the surface of a bacterium?
- Aa glycoprotein in the cell surface membrane
- Ba molecule of glucose in the cytoplasm
- Ca molecule of ATP in the cytoplasm
- Da phospholipid in the inner layer of the membrane
What is meant by a non-self antigen?
- Aan antigen that is not normally found on the body's own cells, so it triggers an immune response
- Ban antigen made by the body's own cells that the immune system ignores
- Can antibody that binds to foreign cells in the blood
- Da pathogen that has entered the body through a wound
A child has an inherited condition in which neutrophils cannot carry out chemotaxis. Which effect is most likely?
- Aneutrophils fail to move to sites of infection, so bacterial infections spread more easily
- Bneutrophils engulf bacteria but cannot digest them
- Clymphocytes can no longer make antibodies against bacteria
- Dlysosomes cannot fuse with phagosomes, so bacteria grow inside neutrophils
Some bacteria are surrounded by a thick slippery capsule that hides their surface antigens. These bacteria are much more likely to cause disease than strains without a capsule. What is the best explanation?
- Areceptors on phagocytes cannot bind to the hidden antigens, so the bacteria are not engulfed
- Bthe capsule contains enzymes that digest the lysosomes of phagocytes
- Cthe capsule makes the bacteria divide faster than other strains
- Dphagocytes recognise the capsule as a self antigen and protect the bacteria
A patient with a bacterial infection had a blood test. The neutrophil count was 12 000 per mm3; the normal range is 2000 to 7500 per mm3. By how much does the count exceed the upper limit of the normal range, as a percentage of that limit?
- A60%
- B160%
- C38%
- D500%
Quiz key
Neutrophils are short-lived phagocytes with lobed nuclei that leave the blood in large numbers at the start of an infection. Long-lived cells that present antigens are macrophages. Antibody secretion is the job of plasma cells, derived from B-lymphocytes, not phagocytes. Neutrophils engulf more quickly when pathogens are coated with antibodies (opsonisation), but they can engulf uncoated pathogens too.
Lysosomes are vesicles containing hydrolytic enzymes such as lysozyme; they fuse with the phagosome and release the enzymes onto the bacterium. Rough ER and ribosomes make these enzymes in the first place, but they do not fuse with the phagosome. Mitochondria supply ATP, not digestive enzymes.
P has a lobed nucleus, the mark of a neutrophil. Q is the largest cell with a single rounded nucleus and many vesicles (lysosomes), a macrophage. R is small with a large round nucleus filling almost the whole cell and little cytoplasm, a lymphocyte. Swapping P and Q ignores the lobed nucleus; calling R a neutrophil or macrophage ignores the very thin rim of cytoplasm typical of lymphocytes.
Antigens are usually proteins or glycoproteins (sometimes glycolipids or polysaccharides) exposed on the outer surface, where lymphocyte receptors and antibodies can bind them. Glucose and ATP are small, common molecules inside the cell, found in our own cells too. Phospholipids in the inner layer are not exposed and are not specific enough to act as recognisable markers.
Non-self antigens are foreign to the body, for example on pathogens, transplanted tissue or wrongly matched blood, and they stimulate an immune response. Antigens on the body's own cells are self antigens, which are tolerated. An antibody is the protein made in response to an antigen, not the antigen. A pathogen is the whole organism; it carries many antigens.
Chemotaxis is movement towards chemicals released by pathogens and damaged cells, so without it neutrophils do not reach infected tissue and bacteria multiply unopposed. Digestion and lysosome fusion happen after engulfing, which needs the neutrophil to arrive first; these steps themselves are not the ones affected. Antibody production by plasma cells is a separate process that does not depend on neutrophil movement.
Attachment to a pathogen depends on phagocyte receptors binding exposed non-self molecules. If the antigens are hidden, binding fails and engulfing is much less effective, so the bacteria survive. The question says nothing about enzymes or growth rate; these are invented mechanisms. A capsule is not a self antigen; phagocytes do not protect pathogens, they simply fail to recognise them.
Excess = 12 000 − 7500 = 4500; 4500 / 7500 × 100 = 60%. 160% is the count as a percentage of the limit, not the excess. 38% divides the excess by the patient's count (4500 / 12 000) instead of by the limit. 500% compares with the lower limit of 2000 (10 000 / 2000). The rise reflects increased neutrophil production by the bone marrow during infection.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.