Antigens and phagocytosisAQA A-Level Biology: Revision notes
Section 1
Cell surface molecules and self versus non-self
Every type of cell has specific molecules on its surface, many of them proteins, which identify it. The immune system uses these to recognise self (the body's own healthy cells) and non-self (foreign or abnormal cells). It identifies:
- pathogens, such as bacteria and viruses
- cells from other organisms of the same species, for example transplanted tissue, which is why donors and recipients are matched
- abnormal body cells, such as cancer cells and virus-infected cells
- toxins, produced by pathogens
Transplant rejection is an immune response to donor cell surface proteins. A better tissue match means fewer foreign antigens.
Section 2
Antigens
An antigen is a molecule, usually a protein, that stimulates an immune response. Antigens are found on the surface of pathogens, foreign cells, abnormal cells and in toxins.
The immune response to an antigen is specific: it depends on a receptor or antibody with a shape complementary to that antigen.
An antigen is not an antibody. Antibodies are made by plasma cells in response to antigens.
Section 3
Antigen variability
Some pathogens show antigen variability: mutations in the genes for their antigens give new strains with antigens of a different shape.
- Effect on disease: memory cells and antibodies from a previous infection are not complementary to the new antigens, so a person can be infected again. Influenza and HIV are examples.
- Effect on prevention: vaccines have to be updated, for example a new seasonal influenza vaccine each year, and it is hard to predict which strains will be common. Developing vaccines against variable pathogens such as HIV is very difficult.
Link variability to memory cells: the old memory cells have the wrong complementary shape, so there is no fast secondary response.
Section 4
Phagocytosis of pathogens
Phagocytes such as neutrophils and macrophages ingest pathogens by phagocytosis.
- A pathogen releases chemicals that attract the phagocyte, which recognises and attaches to antigens on the pathogen's surface.
- The phagocyte's membrane surrounds and engulfs the pathogen, forming a vacuole called a phagosome.
- Lysosomes fuse with the phagosome and release lysozymes (hydrolytic enzymes).
- Lysozymes hydrolyse the pathogen, for example breaking down bacterial cell walls, and the useful products are absorbed into the phagocyte's cytoplasm.
Phagocytosis is non-specific: it happens for many different pathogens.
Lysosomes are the organelles; lysozymes are the enzymes they contain. Do not mix them up.
Section 5
Applying the ideas
Questions often give a scenario: a transplant, a new influenza strain or a patient whose phagocytes fail to destroy bacteria. Work out which step has failed. If bacteria are engulfed but survive, the problem is after engulfment, in lysosome fusion or lysozymes. If a vaccinated person falls ill, think antigen variability and non-complementary memory cells.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Antigens and phagocytosis
- Influenza viruses carry two types of glycoprotein on their surface, haemagglutinin and neuraminidase. The genes coding for these proteins mutate frequently. Every year public health scientists predict which strains will be common and a new seasonal influenza vaccine is produced.Explain why a new influenza vaccine has to be produced each year.2 marks
- A patient with kidney failure receives a kidney from an unrelated donor. Without drugs that suppress the immune system, the patient's immune system would attack the transplanted kidney.Explain why finding a donor with a very close tissue match reduces the risk of the kidney being rejected.2 marks
- Neutrophils, a type of phagocyte, were mixed with Staphylococcus aureus bacteria and viewed under a microscope. After a few minutes bacteria could be seen inside membrane-bound vacuoles in the neutrophils. A rare inherited condition causes a patient's neutrophils to engulf bacteria normally, but the bacteria survive inside the neutrophils and the patient suffers repeated infections.Describe how a neutrophil ingests and destroys a bacterium.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).