2. Antigen processing & Presentation Flashcards
What are the three major classifications of cell types involved in the immune response?
- Polymorphonuclear leukocytes PMNs (granulocytes)
- Monocytes
- Lymphocytes
Which cell type can be considered the first line of defence against an invading pathogen?
PMNs
- Neutrophils in particular
- Drawn towards infection by chemotaxis
Describe PMNs…
- Can discriminate between self and nonself
- Not programmed to ingest specific antigens
- Ingestion is facilitated when the antigen is coated by antibody or by complement protein
- Don’t possess immunological memory
What are the two types of T helper cells and what do they do?
Th1: Promotes cell mediated immunity
Th2: Promotes antibody production
Describe Neutrophils…
- Lifespan of only a few days
- Contain primary and secondary granules in their cytoplasm
- Possess many receptors on the cell surface
- Receptors for Ig
- Capture foreign material through phagocytosis
Describe the process of Neutrophils phagocytosing…
- Chemotaxis
- Adherence & Opsonization
- Ingestion
- Digestion
What is the fate of neutrophils?
- They are capable of undertaking only a limited number of phagocytic events
- After phagocytosis many undergo apoptosis and are removed by macrophages
What is Opsonization?
Coating of a particle with molecules that promote phagocytosis
- molecules are called opsonins
What are the three major types of PMNs?
- Neutrophils
- Eosinophils
- Basophils
Describe Eosinophils…
- Second major PMN
- 12 day half life in tissues
- 30s half life in circulation
- Phagocytic cells that can ingest and destroy foreign material
- More effective in the extracellular digestion/ damage of infectious agents
- Contain two types of granules
- Possess surface receptors for immunoglobulin and complement
What is the major function of Eosinophils?
The destruction of parasitic helminths
- Antibody coated parasites attach to the eosinophil which degranulates and releases its granule contents into the worm cuticle
Describe Basophils…
- PMN
- Found primarily circulating in the blood
- Contains a complex of vasoactive molecules
- Have high affinity IgE receptors on their cell surface
What are lymphocytes…
Cells that originate from bone marrow and lymphoid stem cells
What are the primary lymphoid organs?
Organs that regulate the production and differentiation of lymphocytes
- Thymus
- Bursa (birds)
- Peyer’s patches
- Bone marrow
What are the secondary lymphoid organs?
Tissues where antigen processing occurs and where lymphocytes respond to antigen
- Tonsils
- Spleen
- Peyer’s patches
- Bone marrow
How can B cells and T cells be differentiated from each other?
By their surface components (antigens and receptors)
Describe a B cell receptor…
BCR
- Consists of antibody molecules bound to the cell membrane in such a way that they are able to bind antigen to the B cell surface
- Can bind to free antigen molecules
Describe a T cell receptor…
TCR
- Not an immunoglobulin molecule
- Always associated with a protein called CD3
- Found on all T cells and not on any other cell type
- Can only bind to processed antigen that is associated with MHC molecules on the surfaces of APCs
CD4+8-
T helper cell
- CD4
- Involved in positive regulation of the acquired immune system
- Without them, the immune response to a particular foreign antigen would be weak/ non-existent
- Able to communicate with other cells of the immune response and activate/ regulate them by cytokine release
- TH1 and TH2 cells secrete different sets of cytokines
CD4-8+
Cytotoxic T cell
- CD8
- Lytic capability
- Important for the recognition and elimination of virus infected and tumour cells
- Requires antigen to be presented to them by MHC I molecules on other cells
Describe TH1 cytokines…
Key mediators of cell-mediated immunity
e.g. macrophage activation and delayed-type hypersensitivity
Describe TH2 cytokines…
Stimulate B cell proliferation and immunoglobulin secretion
What are regulatory T cells?
T cells that are involved in controlling the intensity of the immune response
- Secrete cytokines that turn down the immune system
What is a Natural T reg cell?
T cells that originate in thymus and are present in the host before pathogen exposure
What is an Inducible Treg cell?
T cell produced in the secondary lymphoid tissue and acquire regulatory functions in the context of a given infection
What are natural killer cells?
A subset of lymphocytes that arise from precursor cells in the bone marrow
- Present in blood, bone marrow and spleen
- Defense against viral infection
- May have a role in immune surveillance to protect against metastasis of tumours
What are the three major routes an antigen can follow once it gains access to the body?
- Access via the bloodstream
- Access via epidermal, dermal or subcut tissue
- Access via GIT or respiratory tract
Describe monocytes…
- Develop from immature hematopoietic progenitor cells in the bone marrow
- Progenitor cell is the monoblast which develops into a promonocyte then into a monocyte
- Mature monocytes enter the circulation
Where are MHC I found?
On the surface of most nucleated cells (including APCs)
Where are MHC II found?
Only on the surface of lymphocytes, dendritic cells an macrophages
Where are MHC III found?
Not found specifically on cell surfaces
What is a thymus dependent antigen?
Molecules that are composed of repeating units that can directly trigger the antibody producing B cell to produce IgM
When does the T helper cell recognise an antigen?
Only when it is together with MHC antigens on the surface of the APC
- Ensures no self antigens evoke an immune response
What happens once an antigen has been recognised?
the antigen is fragmented in an acidic compartment that is produced when the endosome fuses with the lysosome
- Here the individual epitopes from the antigen are thought to become associated with MHC II
What are the three major functions of dendritic cells?
- Act as sentinel cells and activate immune responses
- Process exogenous antigen for presentation to T cells
- Direct the emphasis of the immune response to be predominantly antigen mediated or cell mediated
What are plasmacytoid dendritic cells?
Found in blood, bone marrow and lymphoid tissue
- derived from lymphoid precursor
What are myeloid dendritic cells?
Differentiate from blood monocytes
- Derived from myeloid precursor
What are follicular dendritic cells?
Found in secondary lymphoid organs
- Derived from myeloid precursor
What are langerhaans cells?
Found in epidermis and mucous membranes
- Derived from myeloid precursor
- They can pick up and process antigen and travel to lymph nodes where they present antigen to activate T cells
What does a T helper cell require for it to become activated?
- Binding of TCR to MHC II on the APC
- Cytokines
What happens when T cells kill?
They exocytose granules that release their contents (cytotoxins) into the intracellular space between the target cell and the effector
- Among granule contents are proteases which degrade proteins
- Apoptosis can also be triggered
What is the sequence of events that occur to initiate the antibody production by plasma cells?
- T helper cell activated following antigen presentation by APC
- Relevant B cells are engaging antigens through their immunoglobulin receptors
- Antigen-receptor complex is drawn into the cell by endocytosis
- Full activation of B cell is achieved when cytokine signals are provided by a T helper cell
- B cells also function as APCs
- Effector T cells can then deliver the appropriate cytokines that can affect other T and B cells that are in close proximity
