Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
10 Cards in this Set
- Front
- Back
How does viruses infect cells?
|
Virus must enter cells in order to replicate
-To enter the cell, virus must first bind to a receptor on the surface of cell -Many viruses take over the cells metabolic resources and use the machinery in order to make copies of themselves -Progeny viruses are released from the first infected cell and then bind and infect a neighbouring cell and the cycle is repeated |
|
How does the immune system response to viral infections?
|
Some of the virus particles that enter the body of a host will bind to and infect cells and begin to replicate
-APC cells may phagocytose other virus particles -B cells that have mIg that recognize epitopes on viral proteins may bind to the virus particles that have entered the lymph node or spleen -Immune response that develops following a viral infection includes both cell-mediated immunity involving cytotoxic T cells and humoral (antibody) immunity |
|
How does cell mediated immunity fight off viruses?
|
-Antibodies can neutralize viruses and prevent them from infecting cells
-Antibodies are very effective against viruses that spend a signficiant amount of time outside of cells -For exmaple, poliovirus which must migrate through the blood from its initial target cells in the gut to its final target cells in the central nervous system -However they are not very effective against viruses that quickly infect cells and then remain in that cell for long periods of time (herpes virus) -In this case cytotoxic T cells are more important -CTLs kill the viral infected cell and help to eliminate the virus by disrupting its replication cycle |
|
In dealing with viruses, when are cytotoxic cells more effective?
|
When dealing with viruses that quickly infect cells and remain in that cell for long periods of time
|
|
In dealing with viruses when are antibodies most effective?
|
When the virus spends a significant amount of time outside the cell because antibodies are too large to diffuse into cells
|
|
After a virus has infected a host cell, viral proteins are made in cytoplasm of the host cell as part of the virus replication cycle. What happens next?
|
Some of these proteins are broken down by the proteasome into peptides in the cytoplasm
-The peptides are transported into the ER by the TAP transporter proteins -In lumen of the ER peptides bind to MHC class I proteins -Peptide/MHC class I complex is transported to cell surface -Antigen presneting cells would phagocytose some viruses -After processing, viral peptides are displayed on MHC class II proteins on the cell surface |
|
Then what?
|
T helper cells are activated when TCR binds to viral peptide-MHC II complex of the APC and CD28 binds to the B7 costimulatory molecule
-In early stages of a viral infection, some of the CD4 T cells receieve cytokines from the APCs (IL-12 and IFN-gamma) that direct the T helper cell to differentiate into a Th1 cell -Th1 cells are important in the activation of CTLs by providing a cytokine called IL-2 |
|
Then what?
|
CTL-precursors CTL-P are incapable of killing target cells because they do not synthesize granules containing perforin and granzymes
-Perforin forms pores in the membrane of the target cell -Granzymes are a set of enzymes that initiate apoptosis (programmed cell death) in target cell -Activation of CTL-P into a functional CTL with cytotoxic activity requires at least three sequential signals 1) Signal transmitted by TCR/CD3 complex after recognition of peptide MHC class I complex on an APC 2) Co sitmulatory signal transmitted by the interaction of CD38 and B7. 3) Il-2 produced by the CTL by activated CTL-P or by activated TH1 cells drive proliferation Often the amount of IL-2 produced by CTL is not enough to allow for much proliferation. More IL-2 can be provided by the T helper cell |
|
Then what?
|
Some of progeny of the CTL-P cells become memory cells
-Others become activated cytotoxic T cells -activated CTL are ready to kill their targets and require only one signal provided by the recognition of the viral peptide presetned by MHC class I on any infected cell -The TCR of activated CTL cells bind to cells displaying the same peptide/MHC class I complex sending a signal to the inside of the CTL -This signal causes the CTL to make both secreted and cell surface proteins that kill the infected cell -Killing requires cell to cell contact |
|
What happens after the killing of an infected cell?
|
THe CTL can release this dead cell and kill other infected cells displaying the same viral petide-MHC clss I complex
-Non infected cells are not killed because they do not have this viral peptide bound to MHC class I protein on their surface -Killing is antigen specific |