1 Flashcards
What is the importance of the immune system?
Complex cellular and protein network that has evolved to protect the host from pathogenic microbes
- Also involved in homeostasis e.g. tissue repair
- Failure of the immune system (immunodeficiency e.g. Severe Combined ImmunoDeficiency) leads to increased susceptibility to infection (and sometimes cancer)
- Abnormal function or regulation of the immune system may lead to disease, e.g. autoimmune diseases, allergy
WHAT is the normal function of the immune system?
To identify and eliminate harmful microorganisms and harmful substances such as toxins.
•by distinguishing ‘self’ from ‘non-self’ (sometimes ‘normal self’ from ‘abnormal self’ in cancer)
and
•by identifying ‘danger’ signals
•The immune system must strike a balance between clearing the pathogen and causing collateral damage to the host - “immunopathology”
What is strategy 1 that Immune system cells use receptors to recognise danger?
Strategy 1 uses Pattern Recognition Receptors (PRRs)
Two types of triggers
•PAMPs – Pathogen-Associated Molecular Patterns (DNA, RNA, peptidocglycans etc.)
•DAMPs – Damage-Associated Molecular Patterns (normally inside cells but released during apoptosis e.g. ecm components)
What is strategy 2 that Immune system cells use receptors to recognise danger?
The B cell antigen receptor is a membrane-bound antibody
- binds intact antigens.
Expressed on the T cell surface are 2 protein chains (α and β) which together make the T cell receptor (TCR)
- binds ‘processed’ antigen fragments presented at cell surfaces
Each antigen receptor binds to a particular site – an epitope – on a different antigen.
Each clone of B cells or T cells has a unique receptor, specific to one antigen; each cell carries many copies of this receptor on the cell surface.
Compare strategy 1 and 2 for cells recognising danger
Strategy 1
•Molecular patterns
•Hundreds of receptors
•Germ-line encoded
- Strategy 2
- Precise molecular structure
- Millions of receptors
- Generated by random recombination of gene segments
Describe the innate response
Innate (uses strategy 1)
- independent of previous exposure
- depends on pre-formed and rapidly synthesised components
- fast (starts in minutes/hours)
• limited specificity –
pattern recognition
of ‘danger signals’
Describe the adaptive response
Adapative (uses strategy 2)
•an adaptation to exposure (also called acquired immunity) •depends on clonal selection: i.e. expansion of cells or antibodies, selected for antigen specificity
• slow (starts in days)
• highly specific to foreign
antigens
What is the function of the innate immune response?
Destroys invading nucleic acids (e.g. viruses) in the cytoplasm
• Activates interleukins which, in turn, activate inflammatory pathways
• Evokes type 1 interferons - bind to receptors on nearby cells and induces an
antiviral state.
• Two main consequences:
Suppresses pathogen replication
Buys time
Kick starts the acquired immune response
How is diversity generated in lymphocytes?
During B and T cell development, random genetic recombinations occur within each cell among multiple copies of immunoglobulin gene segments (B cells) or TCR gene segments (T cells). =strategy 2.
These processes generate the diversity of clones of lymphocytes: each clone is specific to a different antigen.
Describe clonal selection
Antigen binds to surface receptor on the B cell (Ig) or the T cell (TCR) and causes selective expansion of that clone.
• Most lymphocytes that have proliferated will DIE after fulfilling their function
• Some lymphocytes survive as memory cells
What is an epitope?
Epitope - the part of the antigen that is bound by the antibody
Define antigen, immunogen and antibody
Antigen - a molecule that reacts with antibodies or T cells.
• Immunogen - antigens that initiate an immune response.
• Antibody - an immunoglobulin molecule in the blood and body fluids, which binds specifically to an antigen.
Define active and passive immunity
“Active immunity is the induction of an immune response within an individual by the introduction of antigen.
Passive immunity is immunity gained without antigen induction of a response i.e. by transfer of antibody, immune serum or activated lymphocytes into a naïve recipient.
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