2.4 Cell Recognition And The Immune System Flashcards
What is a pathogen?
Pathogens are organisms that cause disease.
What can immune cells detect?
- Cells from other organisms
- Abnormal body cells
- Toxins (produced by pathogens)
What signals to the immune system that a cell is ‘foreign’?
Its antigens
Describe the process of phagocytosis
Foreign antigens bind to specific receptors on the cell surface of phagocytes.
The phagocyte moves towards the pathogen.
The phagocyte cytoplasm surrounds the pathogen and the pathogen is engulfed.
When the pathogen is engulfed, it is sealed into a phagosome (a vacuole) inside the cytoplasm.
Phagocytes have many organelles called lysosomes that contain proteolytic enzymes.
A lysosome fuses with the phagosome and releases the proteolytic enzymes (lysozymes) into the phagosome.
The enzymes break down the pathogen.
The pathogen antigens are transported to the phagocyte cell membrane and presented on the cell surface.
The antigens can activate the other cells in the immune response.
What immune response involves T cells?
Cellular response
What immune response involves B cells?
Humoral response
Describe the cellular immune response
The foreign antigens presented by phagocytes bind to specific receptors on the cell surface of T lymphocyte cells.
Binding of the antigens activates the T helper cells.
T cytotoxic cells are activated by T helper cells.
T cytotoxic cells release toxins that bind to and kill foreign cells and abnormal cells in the body.
T helper cells also activate B cells which activates the humoral immune response.
Describe the humoral immune response
T helper cells activate B cells.
The selection of the correct B cell with a complementary receptor to a pathogen’s antigen is called clonal selection.
Once the correct B cell has been selected, it divides multiple times to produce many identical plasma cells. Plasma cells make antibodies against specific antigens. This process is called clonal expansion.
The antibodies are called monoclonal antibodies.
Monoclonal antibodies bind to the antigens of the pathogens.
Agglutination ‘clumps’ the pathogens together.
The pathogens are engulfed by phagocytes via phagocytosis.
The pathogens are destroyed.
Describe the structure of antibodies
Each antibody has two variable regions.
Each antibody has different variable regions.
The variable regions bind specifically to specific antigens.
One antibody can bind two antigens. This allows the antigens to be clumped together in agglutination.
Antibodies also have constant regions which are the same for all antigens.
Antibodies are made from two heavy chains and two light chains.
The heavy chains are connected to the light chains by disulphide bridges.
The variable region is connected to the constant region via a hinge protein.
What is a primary immune response and what are the features of it?
The response when an individual is infected with a pathogen for the first time. The primary response is slow because it takes time for the antigens to be detected and the specific plasma cells to be activated. T cells and B cells can produce memory cells after an infection.
What is a secondary immune response and what are the features of it?
A secondary immune response is when an organism is infected by a pathogen for the second time. The secondary response is rapid because of the presence of memory cells in the body.
The memory cells can quickly identify the antigens and produce the correct antibodies. The secondary immune response is also stronger than the primary immune response.
More plasma cells can be produced more quickly. Symptoms of the pathogen are unlikely to be shown.
How do vaccinations work?
Vaccines insert the antigens for a specific disease into the individual. The presence of antigens in the body induces the primary immune response. The antigens are inserted as a dead or weakened pathogen so the vaccine does not give rise to a full-scale infection with symptoms. The primary immune response leads to the production of memory cells. If the individual is infected by a living pathogen, the memory cells will be ready to induce a fast, strong secondary response.
What is herd immunity?
Herd immunity works if a large proportion of the population is vaccinated against a disease.
If most of the population are immune, the chance of someone without immunity coming into contact with the disease is reduced.
What is active immunity? Give examples
Active immunity is where the immune system has created its own antibodies.
Active immunity can be:
Natural (antibodies have been created in response to catching a disease).
Artificial (antibodies have been created in response to vaccination of antigens).
What is passive immunity? Give examples
Passive immunity is where an individual receives antibodies from an external source. The antibodies are not made by the individual.
Passive immunity can be:
Natural (antibodies are transferred to a baby from its mother in breast milk).
Artificial (antibodies are transferred to an individual by injection).
Structure of HIV
Capsid contains RNA and reverse transcriptase. The capsid is enclosed by a viral envelope. The viral envelope has glycoproteins on its surface. The glycoproteins bind to the cell surface membrane of T helper cells to infect the cells.
How does HIV replicate?
The glycoproteins on the surface of HIV binds to receptor proteins on the cell surface membrane of T helper cells. The capsid is injected into the T helper cells. The RNA and reverse transcriptase are released from the capsid. Reverse transcriptase is an enzyme that converts the single-stranded RNA from the HIV into double-stranded DNA. The DNA moves into the nucleus of the T helper cells. Viral proteins are synthesised and HIV molecules are assembled then released from the T helper cell.
How is AIDS acquired?
When a HIV virus infects a T helper cell and replicates, the cells are killed.
As HIV spreads through the body and more T helper cells are killed, the immune system weakens.
The weak immune system makes the individual highly susceptible to infection by other pathogens. This is AIDs.
How can monoclonal antibodies be used?
- Targeted medication
- Medical diagnosis
Describe an ELISA test
The antigen for the infection that is being tested for is fixed to the bottom of a beaker. A sample of the blood that is being investigated is added to the beaker. If the antibodies that are complementary to the disease-causing antigen are present in the blood, they will bind to the antigens at the bottom of the beaker to form antigen-antibody complexes. The beaker is washed out so that any antibodies that have not bound to antigens are removed. Secondary antibodies are added to the solution. The secondary antibodies are bound to an enzyme. The beaker is washed again to remove unbound secondary antibodies. The solution that reacts with the enzymes on the secondary antibodies is added. If the solution changes colour, the secondary antibodies have bound to the antibodies from the blood sample so the blood sample is infected by the pathogen.
