Immunology Flashcards
Immune response
When the body is invaded by any pathogen, a series of responses ensure that the pathogen is rapidly identified (as non self), and destroyed before too much damage is caused to the affected organism.
Each type of cell has specific molecules on its plasma cell-surface membrane 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
Antigen:
An antigen is a molecule (usually a protein) that stimulates an immune response that results in the production of a specific antibody. (Antibody generator)
The surface of all own cells (self) and foreign cells or pathogens (non self) are covered in specifically shaped antigens. These antigens help identify each particular type of cell to the host organism.
Therefore, if the antigens are not recognised, the body will treat that cell/pathogen as non-self and initiate an immune response which will lead to the destruction of the cell / pathogen / protein.
Glycoproteins & Glycolipids are both examples of antigens.
Phagocytes:
Phagocytes are a groups of white blood cells which can distinguish between cells which do or do not display the self-antigens
Phagocytes will ingest / engulf and destroy any cell that presents a non-self-antigen. This process is called phagocytosis.
This process is non-specific and works the same for any cell / pathogen that displays a (non-self) antigen. However, it would take far too long to destroy all the invading pathogens in the event of an infection, which may result in damage to tissues and organs. As a result, we also have more efficient systems which involve specific immunity.
Phagocytes are also capable of detecting chemical signals produced by pathogens. Early in an infection, this group of white blood cells move to the site of infection, engulf and begin to hydrolyse pathogens
Non-Specific Immunity
1 + 2) Pathogen is engulfed by the phagocyte
3) Engulfed pathogen enters the cytoplasm of the phagocyte in a vesicle which is now called a phagosome.
4) Lysosomes fuse with phagosome releasing hydrolytic digestive enzymes (lysozymes)
5) Lysosome enzymes hydrolyse the pathogen
6+7) Waste materials are released from the cell by exocytosis and antigens presented on the cell surface membrane and the phagocyte becomes an antigen presenting cell
Specific Immunity
A specific response to a specific antigen on the surface of a cell or pathogen that has been recognised as non-self
T Lymphocytes – Cell-mediated immunity (Primary response)
T Cells are responsible for the stage of an immune response called the cellular response.
The cellular response occurs in the following stages:
1) Antigen presenting
2) Clonal selection
3) Role of T cells
1) Antigen presenting
T (Helper) cells can respond directly to the (specific) pathogen or its antigens or they respond to antigen presenting cells, that presents the specifically complementary antigen.
The antigen presenting cell presents the pathogen’s antigen on its cell surface membrane.
Antigens can also be presented by infected body cells, not just phagocytes or pathogens. T helper cells can also detect the faulty antigens on abnormal cells (e.g. cancer cells)
Cellular Response:
1) Phagocyte engulfs & hydrolyses the pathogen and presents the antigen on the cell surface membrane
2) T Helper cell with specific receptor molecule binds to presented antigen
3) Once T Helper cell binds to the presented antigen it is activated. It then rapidly clones by mitosis.
2) Clonal selection
a) A specific TH cell binds to presented antigen via its complementary receptor.
b) TH cell is activated and clones to produce many TH cells with complementary receptors to the antigen.
This method is required as there would not be enough room in the body to have lots of every T cell for every antigen you may encounter. The increased number of cells would increase the total energy demands of the organism.
3) Role of T cells
When the specific TH cell has been activated, the cloned daughter cells differentiate into 2 different types of T cell
1) T Helper cell
2) Cytotoxic Killer T cell
T Helper Cell
1) Specific TH cell binds to the antigen presenting cell
2) Release cytokines that attract phagocytes to the area of infection.
3) Release cytokines that activate Cytotoxic Killer T cell
4) Activates a specifically complementary B cell
5) Form memory TH cells
Cytotoxic Killer T Cell
1) Locates and destroys infected body cells that present the correct antigen
2) Binds to antigen-presenting-cells
3) Releases perforin (protein) which creates holes in the cell surface membrane which destroys the Antigen Presenting Cell
B cells - Humoral Response (Primary response)
The humoral response involves the activation of B cells to produce antibodies.
B cells must be stimulated by their complementary TH cell by the release of cytokines.
B Cell Activation
1) A specific T Helper cell with the correct receptor binds to presented antigen and then locates AND activates a specifically complementary B cell. The specific T Helper releases cytokine chemicals that signal the specific B cell to clone by mitosis (clonal selection)
2) The B cell then differentiates into two types of cell:
a) Plasma cells -
Produce and secrete vast quantities of specific antibodies into the blood plasma
b) Memory (B) cells -
Remain in the body to respond to pathogen rapidly and extensively should there be a future re-infection
The whole process from initially recognising a pathogen as non self, up to producing antibodies is called the Primary response.
Antibodies:
Protein made in response to foreign antigen – has binding sites which bind specifically to an antigen. A specific antibody is produced by a specific ‘Plasma cell’
Antibodies are complex proteins with a quaternary structure (4°), made up of four polypeptide chains. The overall shape of the antibody is “Y-shaped”
Antibody Structure:
-The main part of the antibody is the same in all antibodies this is the constant region;
-The variable regions have a different primary structure and therefore a different tertiary structure (different shapes);
-It is the variable region and therefore binding site that is specific which is different for each antibody;
-Specific antibodies are only complementary to one antigen
-Due to their specific binding sites, antibodies only bind to specific antigens forming a (permanent) antigen-antibody complex.
How do antibodies assist in the destruction of pathogens?
1) AGGLUTINATION: Specific antibodies bind to the antigens on pathogen and clump them together.
2) OPSONISATION: marking pathogens so phagocytes recognise and destroy the pathogen more efficiently.
3) LYSIS: Bind to antigens and lead to destruction of the pathogen’s membrane.
4) ANTI-TOXIN & ANTI-VENOM: Bind to toxins or venom (both usually proteins) to prevent these molecules from binding to their complementary target receptors.
5) Prevent pathogen replication
Memory cells
As well as producing plasma cells and antibodies, B cells also produce memory cells as part of the humoral response. These memory cells are not involved directly in destroying the invading pathogen.
The role of the memory cell is to remain in the circulation in case of future re-infection by the same pathogen.
Memory Cells:
1) If the memory cells encounter the antigen again, they are rapidly activated (by cytokines secreted by specific TH cell) and divide rapidly by mitosis.
2) The genetically identical cloned memory cells differentiate into plasma cells and more memory B cells.
3) The plasma cells produce lots of the specific antibodies for the invading pathogen, in a short period of time.
Secondary response:
The activation of memory cells to produce antibodies.
The secondary response is both RAPID and EXTENSIVE.
The antigen is normally eliminated before it can cause disease or any symptoms develop
