Within minutes of a cut, white blood cells are crawling towards the bacteria that got in. Phagocytes do not need to know what the invader is: they only need to tell that it is not part of you.
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.
Fig. 1: the four stages of phagocytosis
Section 01
Engulf first, ask later.
Press Start and watch the neutrophil hunt down the bacteria one by one. Follow the STAGE readout as it changes and watch the BACTERIA LEFT counter fall.
Chemotaxis: pathogens and damaged body cells release chemicals; the phagocyte moves towards the higher concentration of these chemicals.
Recognition and attachment: receptors on the phagocyte's cell surface membrane bind to non-self antigens on the pathogen (binding is easier if antibodies already coat it).
Engulfing: the cell surface membrane extends around the pathogen and encloses it in a vacuole called a phagosome (a form of endocytosis).
Digestion: lysosomes fuse with the phagosome and release hydrolytic enzymes such as lysozyme, which break the pathogen down. Soluble products are absorbed into the cytoplasm.
Feature
Neutrophil
Macrophage
nucleus
lobed (several lobes)
large, round or kidney-shaped
where it works
circulates in the blood and squeezes out into infected tissue
develops from monocytes that leave the blood; lives in tissues such as the lungs, liver and lymph nodes
life span and timing
short-lived; huge numbers arrive first and die after digesting pathogens (pus)
long-lived; larger and slower to act
extra role
none: digests the pathogen completely
keeps antigens and displays them on its surface: an antigen-presenting cell for lymphocytes
Examiner trap
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.
Section 02
Self and non-self.
Click each cell in the scene, or use the chips. Compare the colours of the antigens on your own cells, the bacterium, the virus-infected cell and the transplanted cell, then click the lymphocyte.
Antigen: a molecule that stimulates an immune response, leading to the production of antibodies. Most are proteins or glycoproteins on cell surface membranes (the glycoproteins and glycolipids of the fluid mosaic model, topic 4.1); toxins can also act as antigens.
Self antigens: the antigens on your own cells. During development, lymphocytes whose receptors would bind to self antigens are destroyed, so the body tolerates its own cells.
Non-self antigens: antigens not found on your own cells, for example on bacteria, viruses, parasites, cells from another person (a transplant or wrongly matched blood) or cancer cells with altered antigens.
Infected cells: a virus-infected body cell displays viral antigens on its surface next to its self antigens, which marks it for destruction by T-killer cells.
Specific receptors: each lymphocyte has receptors of one shape only, complementary to one antigen. This is what makes the response in the next lesson specific.
Examiner trap
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.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the phagocyte sim, what makes the neutrophil move towards the bacteria?
Quick check+10 XP
In the sim, what happens at the stage labelled "lysosomes fuse"?
Quick check+10 XP
In the antigens sim, the transplanted cell carries purple antigens. Why would the immune system attack it?
Quick check+10 XP
In the antigens sim, why does the virus-infected body cell show orange antigens as well as blue ones?
Section 03
Where phagocytes fit.
The immune response has two parts that work together. Exam answers often ask you to link them.
Phagocytes
Lymphocytes
specificity
non-specific: any non-self material
specific: one antigen per clone
speed
immediate, within hours
days in a first infection (clonal selection and expansion)
memory
none
memory cells give long-term immunity
link
macrophages present antigens that activate T-helper cells; antibodies made by plasma cells coat pathogens so phagocytes engulf them faster (opsonisation)
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which feature distinguishes a macrophage from a neutrophil?
Exam style+20 XP
Which statement correctly defines an antigen?
Exam style+20 XP
Which sequence of events during phagocytosis is correct?
Exam style+20 XP
Why do lymphocytes normally not attack the body's own cells?
Skill unlocked
Phagocytes and antigens, mastered.
This skill is now recorded on your profile. Keep the chain going.