Cell recognition and the immune system Flashcards
(36 cards)
What is immunity?
After the body overwhelms a pathogen, the body’s defences seem to be better prepared for a second infection from the same pathogen and can kill it before it causes any harm.
What are the forms of lymphocytes?
A lymphocyte is a white blood cell and takes two forms:
Cell-mediated responses involving T lymphocytes.
Humoral responses involving B lymphocytes.
Why must own cells be recognised?
To defend the body from invasion by foreign material, lymphocytes must be able to distinguish the body’s own cells and molecules.
(Self from those that are foreign (non-self).
If they could not do this, the lymphocytes would destroy the organism’s own tissues.
What is phagocytosis?
Large particles, such as some types of bacteria, can be engulfed by cells in the vesicles formed by the cell surface membrane.
In the blood, the phagocytes are the types of white blood cells that carry it out.
They provide an important defence against the pathogens that manage to enter the body.
Some phagocytes travel in the blood but can move out of blood vessels into other tissues.
What is the process of phagocytosis?
Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen.
Phagocytes have several receptors on their cell-surface membrane that recognise, and attach to, chemicals on the pathogen’s surface.
They engulf the pathogen to form a vesicle - a phagosome.
Lysosomes move towards the vesicle and fuse with it.
Lysozymes present within the lysosome destroy ingested bacteria by hydrolysis of their cell wall.
The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.
What is an antigen?
Any part of an organism or substance that is recognised as non-self by the immune system and stimulates an immune response.
Antigens are usually proteins that are part of the cell-surface membranes or cell walls of invading cells, such as microorganisms, or abnormal body cells, such as cancer.
The presence of an antigen triggers the production of an antibody as part of the body’s defence system.
What are the types of lymphocyte?
Lymphocytes are produced by stem cells in the bone marrow.
B lymphocytes (B-cells), mature in the bone marrow. They are associated with humoral immunity, immunity involving antibodies that are present in body fluids, or humour such as blood plasma.
T lymphocytes (T-cells), mature in the thymus gland. They are associated with cell-mediated immunity, immunity involving body cells.
Why can T lymphocytes distinguish invader cells from normal cells?
Phagocytes that have engulfed and hydrolysed a pathogen present some of a pathogen’s antigens on their own cell-surface membrane.
Body cells invaded by a virus present some of the viral antigens, on their own cell-surface membrane.
Transplanted cells from individuals of the same species have different antigens on their cell-surface membrane.
Cancer cells are different from normal body cells and present antigens on their cell-surface membranes.
What are antigen-presenting cells?
Cells that display foreign antigens on their surface.
They can present antigens of other cells on their own cell-surface membrane.
What is the cell-mediated immunity/cellular response?
T lymphocytes will only respond to antigens that are presented on a body cell.
The receptors on each T cell respond to a single antigen.
There is a vast number of different types of T cell, each responding to a different antigen.
What are the stages in the response of T lymphocytes to infection by a pathogen?
Pathogens invade body cells or are taken in by phagocytes.
The phagocytes place antigens from the pathogen on its cell-surface membrane.
Receptors on a specific helper T cell fit exactly onto these antigens.
This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells.
What do the cloned T cells do?
They develop into memory cells that enable a rapid response to future infections by the same pathogen.
They stimulate phagocytes to engulf pathogens by phagocytosis.
Stimulate B cells to divide and secrete their antibody.
Activate cytotoxic T cells.
How do cytotoxic T cells kill infected cells?
They kill abnormal cells and body cells that are infected by pathogens, by producing a protein called perforin that makes holes in the cell-surface membrane.
These holes mean the cell membrane becomes freely permeable to all substances and the cell dies as a result.
This illustrates the vital importance of cell-surface membranes in maintaining the integrity of cells and hence their survival.
Why are T cells effective against viruses?
The action of T cells is most effective against viruses because viruses replicate inside cells.
As viruses use living cells in which to replicate, this sacrifice of body cells prevents viruses multiplying and infecting more cells.
Why are proteins the most important cell-surface molecule?
Each type of cell, self or non-self, has specific molecules on its surface that identify it.
While these molecules can be a variety of types, the proteins are the most important.
This is because proteins have enormous cell variety and a highly specific tertiary structure.
It is this variety of specific 3-D structure that distinguishes one cell from another.
What do protein molecules allow the immune system to identify?
Pathogens, for example HIV.
Non-self materials such as cells from other organisms of the same species.
Toxins including those produced by certain pathogens like the bacterium that causes cholera.
Abnormal body cells such as cancer cells.
What are the implications of the cell recognition response?
All of the above are potentially harmful and their identification is the first stage in removing the threat they pose.
Although this response is clearly advantageous to the organism, it has implications for humans who have had tissue or organ transplants.
The immune system recognises these as non-self even though they have come from individuals of the same species.
It therefore attempts to destroy the transplant.
How is organ rejection reduced?
Donor tissues for transplant are normally matched as closely as possible to those of the recipient.
The best matches often come from relatives that are genetically close.
In addition, immunosuppressant drugs are often administered to reduce the level of the immune response that occurs.
What are the types of defence mechanism?
Non-specific: response is immediate and the same for all pathogens.
Physical barrier e.g. skin; phagocytosis.
Specific: response is slower and specific to each pathogen.
Cell-mediated response - T lymphocytes; humoral response - B lymphocytes.
How do lymphocytes respond to an infection?
When an infection occurs, the one type already present that has the complementary proteins to those of the pathogen is stimulated to divide to build up its numbers to be effective in destroying it - clonal selection.
This is why there is a time lag between exposure to the pathogen and body’s defence bringing it under control.
How are lymphocytes stored?
Specific lymphocytes are not produced in response to an infection, they already exist - all 10 million different types.
There are so many different types of lymphocytes, so there is a high probability that when a pathogen enters the body, one of these lymphocytes will have a complementary protein on its surface to one of the proteins of the pathogen.
The lymphocyte will recognise the pathogen.
How do lymphocytes recognise cells belonging to the body in adults?
In adults, lymphocytes produced in the bone marrow initially only encounter self-antigens.
Any lymphocytes that show an immune response to these self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes.
No clones of these anti-self lymphocytes will appear in the blood, leaving only those that might respond to non-self antigens.
How do lymphocytes recognise cells belonging to the body in foetuses?
In the foetus, the lymphocytes are constantly colliding with other cells.
Infection in the foetus is rare because it is protected by the mother and the placenta.
Lymphocytes will therefore collide almost exclusively with the body’s self-material.
Some of the lymphocytes will have receptors that exactly fit those of the body’s own cells.
These lymphocytes either die or are suppressed.
The only remaining lymphocytes are those that might fit foreign material, and therefore only respond to foreign material.
What is the first and second line of defence in the body?
The body’s first defence is to form a physical or chemical barrier to entry.
Should this fail, the next line of defence is the white blood cells - two types: phagocytes and lymphocytes.
Phagocytes ingest and destroy the pathogen by phagocytosis before it can cause harm.
