11.1 Immune system
Lesson one: the Immune response to an infection
Objectives: understand the role of different white blood cells in responding to an infection
- Leucocytes: White blood cells (general term)
- Phagocytes: white blood cells which will ingest pathogens, often displaying one of their antigens on the surface of their cells.
- T-helper cells: A cell which will activate a B-cell, causing it to divide making plasma cells and memory cells
- Plasma cells: White blood cells which make antibodies, Also called B lymphocytes.
- Receptor site: protein on the surface of a B cell which an antigen can bind to, alerting the B cell to the presence of a pathogen.
- Antibodies. Proteins secreted by plasma cells which aid in the destruction of pathogens
- Memory cell: A cell which will respond to the presence of an antigen which has been present before in the system, by stimulating the production of antibodies
- Clonal antibody selection: The formation of a large number of B-cells,all making a specific antibody against a certain antigen.
- Memory trigger: Antigen identifies, Antibody you make, Antibiotic you take to fight disease
Basics of the immune system response
Task – Two introductory videos:
For each one – write down two things that surprised you, and two things you thought were the most important to remember.
The immune system is based on the principle of recognition of cells based on their surface proteins (these are often glycoproteins). The immune system divides the cells encountered into ‘self’ and ‘alien’.
The other principle is that of ‘challenge’ and ‘response’. The response takes place as a result of a ‘challenge’. This challenge takes the form of an antigen present in the body (eg. hemagglutin molecules on surface of influenza virus – the virus that causes the flu, or gripe.).
An antigen is any molecule capable of triggering an immune response. They are often described as ‘foreign proteins’ in the body. Antigens may include surface proteins of red blood cells (a cautionary detail in blood transfusions), the membrane proteins of bacteria such as E. Coli (which may be an intestinal pathogen), a component of a virus protein coat such as hemagglutinin in an influenza virus, or even a wooden splinter trapped in your finger.
A normal response to a ‘challenge’ is to create antibodies which will neutralise the threat.
Antibodies are proteins which are secreted by white blood cells in order to fight diseases. Their distinctive ‘Y’ shape allows them to bind to antigens, causing a number of effects:
- Opsonization (attracting attention): making it easier for phagocytes to recognise pathogens and ingest them.
- Neutralisation (tie them up): Blocking the entry of pathogens into host cells or blocking the effects of toxins on cells
- Agglutination (sticking together): Causing the pathogens to aggregate, making them an easy target and blocking their movement.
- Activation of complement (open and destroy): they intertact with the cell membrane proteins, causing a complex cascade of reactions called a “membrane attack complex”, leading to a hole in the membrane through which water and ions can enter and cause the pathogen to lysis (burst).
An immune response (production of antibodies), step-by-step
There are two ways that antibody production can be stimulated; according to the above:
- Antigen binds to a B cell receptor on a B cell.
- The B cell may also receive more information from a T helper cell about the presence of an antigen. T helper cells may also secrete interleukin (a cytokine, or signal molecule), which stimulates the B cell to become activated.
- Plasma cells can respond by dividing to form many identical plasma cells which will all secret the specific antibody required to fight that pathogen (this is clonal antibody selection).
- Plasma cells can also form memory cells, which will remain after the infection has cleared to provide a more rapid response in the case of re-infection.
In the second route,
A phagocyte ingests a pathogen, and displays it’s antigen on the surface of its membrane. This is detected by T-helper cells, which share the antigen with B cells through surface docking proteins.
Antigens and blood transfusions
Blood groups are caused by the presence of antigens on the surface of red blood cells:
- Group A people have the antigen ‘A’ on the surface of their own red blood cells. This means that they will detect B antigens as foreign invaders and make B antibodies to combat them.
- Group B people have the antigen ‘B’ on the surface of their own red blood cells. This means they will detect A antigens as foreign invaders and make A antibodies to combat them.
- Group O people have neither ‘A’ or ‘B’ antigens on the surface of their own red blood cells. This means they will detect both A and B antigens as foreign invaders and make A or B antibodies to combat them. Since their blood has no antigens, they can donate blood to any other blood group (universal donors).
- Group AB people have both ‘A and ‘B’ antigens on the surface of their one red blood cells. This means that they will not detect A or B antigens as foreign invaders and will not make antibodies to combat them. Since they have both antigens, they can receive blood from any other blood group (universal donors).
Lesson two: Virtual lab (Ebola).
Objectives: Use the case study of the spread of the Ebola virus to apply concepts of epidemiology (the study of the spread of diseases.
- Pathogen: a disease causing agent eg. the Ebola virus.
- Epidemiology: the study of the spread of diseases.
- Core mutation: a mutation shared by all subsequent strains.
- DNA sequence: the sequence of nucleotides (eg. adenine, guanine, cytosine), which makes up the DNA or RNA.
The materials will be provided in class.
This is an introductory video
Background information: Click on Introduction-to-Ebola