2.4 Cell recognition and the immune system Flashcards
Each type of cell has specific molecules on its surface that identify it. These molecules include proteins and enable the immune system to identify:
pathogens
cells from other organisms of the same species
abnormal body cells
toxins
What is an antigen?
Foreign protein / glycoprotein / glycolipid / polysaccharide that stimulates production of antibody.
What is an antibody?
An immunoglobulin protein secreted by plasma cells. Each antibody is specific to an antigen.
What is the effect of antigen variability on disease and disease prevention?
When a pathogen’s genome mutates, it’s antigen structure may change. This means antibodies, and any vaccines designed to stimulate production of those antibodies, become ineffective, and immunity is lost. New vaccines with the new antigen structure would need to be made.
Describe and explain the role of antibodies in stimulating phagocytosis.
Antibodies bind to the antigen, forming antibody-antigen complexes.
Antibodies act as markers (‘opsonins’) which attract phagocytes.
Some antibodies cause agglutination (clumping of several pathogens / toxins together).
Describe how a phagocyte destroys a pathogen.
Engulfs the pathogen by endocytosis, forming a vesicle called a phagosome, which fuses with a lysosome. Hydrolytic enzymes (lysozymes) from the lysosome digest the pathogen.
Describe how phagocytosis of a pathogen could lead to presentation of its antigens.
Some phagocytes are antigen-presenting cells. After the antigen-presenting phagocyte’s phagosome has fused with its lysosomes and the pathogen has been destroyed, the surface antigens from the pathogen are displayed on antigen-presenting phagocyte’s plasma membrane.
Describe how presentation of a virus antigen leads to the secretion of an antibody against this virus antigen.
A Helper T cell binds to the antigen on the antigen-presenting phagocyte. This helper T cell stimulates a specific B cell, which clones by mitosis, then differentiates to form plasma cells that release antibodies complementary to the viral antigen.
Explain why each antibody will only detect one type of antigen.
The antibody’s variable region has a unique amino acid sequence giving its binding site a specific tertiary structure that is only complementary to these antigens and will only form an antibody-antigen complex with this antigen.
Vaccines protect people against disease. Explain how.
Vaccines contain antigens from pathogens. Vaccine particles are engulfed by antigen presenting cells, which then present the antigens on their plasma membranes. Any T cell with a receptor protein that is complementary to the antigen will bind to it. This activated T cell stimulates a B cell which also has a complementary antibody on its surface. The B cell divides an differentiates to form plasma cells and memory cells (the primary response).
If the vaccinated person is then infected with this pathogen their memory cells will bind to the pathogen’s antigen, become active and rapidly produce antibodies which destroy the pathogen population (the secondary response).
There are two types of T lymphocyte (on the spec). What are they and what do they do?
T Helper cells and Cytotoxic T cells. Both are stimulated by antigen presenting cells only (ie not by direct contact with the pathogen).
Stimulated T Helper cells divide by mitosis (cloning themselves) and then release cytokines - cell signalling molecules which stimulate other immune cells including B cells, cytotoxic T cells and phagocytes. Like B cells, some T cells will become memory cells, enabling a rapid response if the same pathogen returned later.
Activated cytotoxic T cells kill their own body’s cells, either because they are abnormal (perhaps because they are part of a tumour) or because they have become infected (usually with a virus). They produce a protein called perforin which makes holes in the target cell’s plasma membrane, making it freely permeable. They may also use hydrogen peroxide, proteases and nucleases.
T cells and B cells respond differently to infection. Their two responses have names. What are they?
T cells are responsible for the cell mediated response.
B cells are responsible for the humoral response.
Describe the structure of antibodies, and explain what is meant by a monoclonal antibody.
Antibodies, or ‘immunoglobulins’, are globular proteins with a quaternary level of structure. They are composed of 4 polypeptide chains, 2 ‘heavy’ (ie longer) and 2 ‘light’ (ie shorter), linked by disulphide bridges. They have two distinct regions; the variable region and the constant region, and between them is a flexible ‘hinge’.
Each B cell produces just one type of antibody, ie all with the exact same variable region. When a B cell undergoes clonal expansion (ie mitosis) ALL the resulting plasma cells will produce the exact same antibody. These are called monoclonal, meaning one type. However each B cell produces DIFFERENT antibodies, ie their variable regions have a different 3D shape to those of other B cells.
The constant region is the same for all antibodies.
What is herd immunity?
Herd immunity is the level of immunity required in a given population to prevent the spread of a particular disease through it - ie the percentage of people that need to be vaccinated to prevent outbreaks of the disease. You don’t need to vaccinate everybody, just enough people that it becomes very difficult for pathogen to pass from one person to another. Different pathogens have differing rates of infection - some spread very easily, others are less infectious - so each disease has its own associated herd immunity percentage. Those that are more infectious need higher rates of vaccination to achieve herd immunity.
Describe the difference between active and passive immunity.
- Active involves memory cells, passive does not;
- Active involves production of antibody by plasma cells / memory cells, whilst passive involves antibody introduced into body from outside (eg in breast milk, or in an injection of anti-venom)
- Active long term, because antibody produced in response to antigen, whereas passive short term, because the given antibody is broken down;
- Active can take time to develop, whereas passive fast acting.
