Key Area 5
(d) Defences against parasitic infection
Immune response in mammals has both non-specific and specific aspects.
Click on the white button to access an excellent introductory video to set the scene for this section. The TED-Ed video below provides another great overview of our immune system.
Non-specific defences
Non-specific defences include:
Physical barriers
Chemical secretions
Inflammatory response
Phagocytes
Natural killer cells destroying cells infected with viruses
Non-specific: 1st line of defence
Physical barriers
All areas of the body are lined with epithelial tissue to form a physical barrier to parasite entry.
Chemical barriers
Hydrolytic enzymes in mucus, saliva and tears destroy bacterial cell walls. The low pH environments of the secretions of the stomach, vagina and sweat glands denatures cellular proteins of pathogens.
Non-specific: 2nd line of defence
Physical and chemical barrier breach...
The inflammatory response kicks in! Click through the slides below to remind yourself what happens during the inflammatory response.
Inflammatory response
Injured cells release signaling molecules. This results in enhanced blood flow to the site, bringing antimicrobial proteins and phagocytes.
Quick self-test
Try this quick matching game to identify the role of each cell during the inflammatory response.
Task 69
Complete the table in your notes using the following factors: histamine, mast cell, cytokine, clotting element, phagocyte. Answers are available here.
Task 70
Complete the flowchart in your notes for the inflammatory response. Answers are available here.
Phagocytes
Phagocytes are white blood cells with a non-specific role in defence. Phagocytes arrive at the site of infection and engulf the parasite by enfolding their plasma membrane around the parasite. The parasite is then brought into the phagocyte in a vacuole or vesicle. Phagocytes contain special organelles called lysosomes. These are filled with digestive enzymes. The lysosomes fuse with the vacuole, releasing the digestive enzymes and allowing the enzymes to digest the parasite.
All your cells have protein surface markers on them. These markers let your immune cells know that your cells are yours and belong in the body.
However, pathogens such as bacteria or viruses also have markers on their surface. But because they do not belong in the body and are ‘foreign’, they are known as surface antigen molecules
•The phagocytes RECOGNISE these foreign markers and engulf them!
Click through the slides below and watch the recommended video to learn more about phagocytosis.
Quick self-test
Try this quick game to put the 6 steps of phagocytosis in the correct order.
Task 71
Immune surveillance - Add a couple of lines your notes about this based on your learning at Higher level.
Phagocytes - complete the paragraph below.
Phagocytes are white blood cells with a ____________ role in defence. Phagocytes arrive at the site of __________ and _______________ the parasite by enfolding their plasma membrane around the parasite. The parasite is then brought into the phagocyte in a _____________. Phagocytes contain special organelles called ____________. These are filled with ________ ______________. The lysosomes fuse with the vacuole, releasing the digestive enzymes and allowing the enzymes to digest the parasite.
Look at the diagram in your notes. What is happening during each stage of phagocytosis?
All your proteins have protein surface markers on them. These markers let your immune cells know that your cells are yours. However, pathogens also have markers on their surface - they surfaces have "surface antigen markers" that mark them as foreign to your body. They are recognised by phagocytes. Label the diagram in your notes.
Arrange the following steps in phagocytosis in the correct order.
The parasite is digested
Parasite is brought into the phagocyte into a vacuole.
Phagocytes move to the site of injury.
Lysosomes fuse with the vacuole releasing digestive enzymes.
Plasma membrane of the phagocyte engulfs the parasite.
Lysosomes move towards the vacuole.
Answers to these tasks can be found here.
Natural killer cells
Natural killer cells are found in the blood and are constantly looking for signs of an infection. These cells can identify and attach to cells infected with viruses, releasing chemicals that lead to cell death by inducing apoptosis.
Watch the 2 short videos below on this subject.
The image above shows a cancer cell under attack by a Natural Killer cell.
Task 72
Natural killer cells have been discussed above. Explain the key points in this diagram.
Answers to these tasks can be found here.
Quick self-test
Try these quick games on non-specific defences,
Specific Defences
Specific: 3rd line of defence
Mammals contain many different lymphocytes, each possessing a receptor on its surface, which can potentially recognise a pathogen antigen.
Binding of an antigen to a lymphocyte’s receptor selects that lymphocyte to then divide and produce a clonal population of this lymphocyte. Some selected lymphocytes will produce antibodies, others can induce apoptosis in parasite-infected cells.
Clonal selection
Plasma cells secrete many copies of antibody into the bloodstream, specific against the foreign pathogen, and seek to destroy it.
Memory cells result in immunological memory - that is, the ability of the body to recognise the antigen if re-infection occurs in the future. Such a "secondary immune response" is faster and stronger, often eliminating the pathogen from the body with limited symptoms and obvious impact on the host.
The diagram (left) shows that when pathogens enter the bloodstream, one B lymphocyte (yellow) will possess a surface receptor specific to the antigens present on the pathogenic cell. Once engaged, clonal selection takes place.
This means this one B-lymphocyte is cloned or copied. Many copies are generated by mitosis and this creates memory cell or plasma cells. Plasma cells produce antibodies to detect and destroy the pathogenic cells; Memory cells provide immunological memory.
The binding of the antibodies causes the:
inactivation of the pathogen (or the toxin it produces)
pathogen to become more susceptible to phagocytosis
How do antibodies work?
Antibodies possess regions where the amino acid sequence varies greatly between different antibodies. This variable region gives the antibody its specificity for binding antigen.
When the antigen binds to this binding site, the antigen-antibody complex formed can result in inactivation of the parasite, rendering it susceptible to a phagocyte, or can stimulate a response that results in cell lysis.
Task 73
Sketch a diagram to highlight the two regions of antibody structure. Answers are available here.
Protection for the future
Memory lymphocyte cells are also formed. Initial antigen exposure produces memory lymphocyte cells specific for that antigen that can produce a secondary response when the same antigen enters the body in the future.
When this occurs, antibody production is enhanced in terms of speed of production, concentration in blood and duration.
Task 74
Read the statements in the table below. Can you identify the correct order of the stages of clonal selection?
This is a great time to consolidate your learning by reading through the material provided by SCHOLAR.
