Adaptive Immunity Flashcards
What are interferons?
They are signalling proteins that are released by host cells.
They stimulate the infected cells to produce proteins that inhibit the spreading of the virus.
What are some differences between the innate and adaptive immunity?
Innate
Diversity: large
Memory: No
Cells: Phagocytes
Adaptive
Diversity: limited
Memory: yes
Cells: Lymphocytes, NK cells
What is the function of dendritic cells?
It is a type of phagocyte that shows antigens on its surface (APCs) to T cells.
They act as messengers.
Which complement protein is the molecular marker for cytotoxic T cells?
CD8
What is the function of cytotoxic T cells, and how do they do this?
They directly kill target cells which are infected (viral/bacterial), and tumour transformed cells.
They do this by releasing perforins to generate pores on the cells surface and induce apoptosis.
They can also induce Fas-mediated cytotoxicity. A Fas ligand is activated and induces apoptosis via the Fas pathway.
Which complement protein is the molecular marker for T helper cells?
CD4
What is the function of T helper cells, and how do they do this?
They stimulate cytotoxic T cells, and help B cells produce and secrete antibodies. This occurs due to:
- Cytokine release
- Binding of CD40L on the T cell to the CD40 receptor on the B cell
- Interactions with TNF-TNF receptor ligands
Which complement protein is the molecular biomarker for B cells?
CD20
What do B cells do?
They differentiate into antibody producing plasma cells with correct stimulus from T helper cells.
Which cell is mainly associated with humoral immunity?
B cells.
What is the function of humoral immunity?
It blocks infections and eliminates extracellular microbes.
Which cells are mainly associated with cell-mediated immunity?
Phagocytes and T lymphocytes (helper/cytotoxic)
What is the function of cell-mediated immunity?
Activate macrophages to kill phagocytosed microbes, and to kill infected cells & eliminate reservoirs of infection.
What do all nucleated cells have?
Molecular marker MHC (major histocompatibility complex)
What is the difference between MHC I and MHC II?
MHC I: is a self-marker protein (expressed on all somatic cells)
MHC II: they present processed antigens on their cell surfaces for T cell recognition.
(Think of MHC II like a ‘serving plate’, where they ‘serve’ the antigen)
What can occur when MHC molecules present antigen contents?
B cell activation, and T cell stimulation.
What is HLA (Human Leukocyte Antigen)?
It is a complex of genes which encodes for cell-surface proteins responsible for the immune system.
What is allorecognition?
It is when the host T cell receptor can distinguish between self MHC I, and non-self MHC.
This can occur from e.g. transplants
Which cells can only bind to MHC on an APC?
T helper cells & B cells.
What is the costimulator in an APC, and what does it bind to on the T helper cell?
Co-stimulator B7 on the APC binds to its co-stimulator ligand CD28 on the T helper cell.
How many T cells are present in lymphoid tissue, and what do they do?
16 T cells are present randomly in lymphoid tissue, and generate an immune response which expels invading pathogens in <6 days.
What are naive T cells?
They are matured precursors for effector/memory T cells, which have not yet encountered an antigen.
How is the adaptive immune response initiated?
Lymphocytes recognise that a cell is either non-self, or contains elements of non-self presented on MHC
Adaptive immune response is then initiated.
What is the whole process of the adaptive immune response, from start to finish?
- A dendritic cell ingests the microbe and releases cytokines to induce cytokine-mediated activation (inflammation).
- The dendritic cell migrates to the lymph nodes via lymphatic vessels
- The dendritic cell matures into an APC and expresses MHC to display the microbial antigen & costimulators (B7)
- Inside the lymph node, the APC presents the antigen to a naive T cell (in the T cell zone) via MHC
- Circulating naive T cells migrate into the lymph nodes and encounter more APCs
- A naive CD4 T helper cell recognises a peptide antigen and costimulator B7 on the APC
- The T helper cell becomes activated and releases cytokines to stimulate its own proliferation and differentiate into effector T cells.
- The activated T cell undergoes clonal expansion (proliferation)
- While an APC is activating a T cell, soluble antigens begin to bind to naive B cells via surface Ig
- MHC type 2 on the B cell binds to the antigen and it begins to migrate to the edge of the follicle.
- Activated T & B cells move towards the edge of the follicle, where B cells present the antigen to the T cell
- The activated B cell proliferates with T cell and cytokine stimulation
- Proliferated B cells then differentiate into antibody secreting plasma cells, and memory cells.
- Plasma cells secrete antibodies that enter the circulation via the efferent lymphatic vessel and target microbes for destruction
- The antibody binds to the microbe and hinders their ability to interact with cell receptors, hence preventing host cell infection
What can free antibodies circulating in the body do?
They can bind to antigens (opsonisation), and allow the microbe to be captured by a phagocyte.
(Remember opsonisation ‘to make food ready for-‘)
What occurs when an opsonised microbe is captured by a phagocyte?
- The Fc region of the antibody binds to the Fc receptors of the phagocyte
- Signals from the Fc receptor promotes phagocytosis of the opsonised microbe.
- Granules are released resulting in the killing of the target cell
What is the process of complement activation?
- A C1 complex binds to 2 or more antigen-bound antibodies (opsonised microbes)
- Complement cascade is activated and a MAC is produced (membrane attack complex)
- Pores are formed in the microbial membrane and ions/water are able to pass through, resulting in osmotic lysis of the microbe
- C3 and C5 fragments from the C1 complex cleave to form C3a and C5a to induce acute inflammation by activating mast cells & neutrophil chemotaxis
