Immune System Flashcards
Define antigen
An antigen is defined as “anything that can be bound by an antibody“
Now antigens are considered any substance that can induce an adaptive/acquired immune response (B or T-cell).
Antigens can be short peptides, proteins, sugars, lipids etc.
Define antibody
Antibodies are proteins produced by adaptive immune cells that bind specifically to relatively small parts of foreign molecules known as antigenic determinants or epitopes.
Define cytokine
Cytokine is a general term to describe various protein molecules secreted by cells of the immune system that serve to regulate the immune system. Can also have effects on other cells in the body.
Often called interleukins (ILs) i.e. IL-1, IL-2 etc. where the number stands for the order of their discovery. Sometimes have other names linked to function Tumor Necrosis Factor alpha (TNF-𝛼).
Define chemokines
Chemokines are a sub-type of cytokines with chemoattractant properties, involved in the movement and migration of immune cells.
What is cluster of differentiation?
CD = cell-surface molecules (generally proteins) on immune cells that are recognized by specific monoclonal antibodies.
example CD1, CD2 etc
Some CDs expressed on specific cell lineages i.e. CD3 is specific to T cells
Role of immune system
Discrimination between self and foreign (with a level of tolerance to non-self)
Protection from pathogens
Handling and neutralization of toxins
Elimination of aberrant cells (cancer cells)
Wound healing
Main functions of:
Innate immunity
Adaptive immunity
INNATE
1. Immune recognition
ADAPTIVE
4. Immunological memory
BOTH
- Immune effector mechanisms
- Immune regulation
Components of innate immunity?
Defining characteristic: Not specific to individual pathogens. No memory persists after encounter.
Physical barriers - skin (dermis) and mucous membranes.
Physiological factors - pH, temperature and oxygen tension limit microbial growth.
Protein secretions – lysozyme, complement, defensins, lactoferrin, cathelicidin etc.
Pro-inflammatory cytokines – Interleukins and chemokines
Phagocytic cells – macrophages and polymorphonuclear leucocytes.
What is adaptive immunity?
Defining characteristic: Immune response directed at specific pathogen and memory persists after initial encounter.
The second level of defence.
Usually increases in strength and effectiveness with each encounter.
Component of the foreign agent (antigen) is recognised in a specific manner and the immune system acquires memory of it.
Instructed by 2 types of lymphocytes T cells and B cells.
Primary lymphoid organs
Where lymphocytes are formed and mature.
- Bone Marrow
- Thymus
Secondary Lymphoid Organs
Monitoring the contents of the extracellular fluids or mucosal surfaces.
- Spleen
- Lymph Nodes
- MALT/GAL
Function of innate immune system
Reacts to microbes (and injured cells
First line of defence (initial response to microbes)
Rapid (immediate => maximal response in hours) It is already in tissues
Prevents, controls, (sometimes) eliminates infection
Innate immune response keeps infection in check and links with the adaptive immunity
Myeloid precursors give rise to…
Monocytes and granulocytes
Lymphoid precursors give rise to…
T cells and B cells
Mechanical barriers
Epithelial cells joined by tight junctions Longitudinal flow of air or fluid Movement of mucus by cilia Tears Nasal cilia
Chemical barriers
Fatty acids Low pH Enzymes (pepsin) Enzymes in tears (lysozyme) Antibacterial peptides
Microbiological barriers
Normal flora
Normal flora displaces pathogenic bacteria
Antibiotics kill normal flora and pathogenic bacteria find a niche
How does cystic fibrosis affect immunological barriers?
Defective mucus production
Inhibition of ciliary movements
= Resulting in frequent lung infections
What are the effector cells of innate immune response
PHAGOCYTES: neutrophils, macrophages and dendritic cells - myeloid lineage generated in bone marrow
MAST CELLS
EOSINOPHILS
BASOPHILS - myeloid lineage
NK CELLS - lymphoid lineage, BM
Describe basophils
Basophils: blue granules (H&E staining)
Few in circulation (blood): 0.2-1% of all WBC
Main role: hypersensitivity type I (allergy)
Degranulation (histamine, peroxidase, heparin, kallikrein) => inflammation
Describe eosinophils
Pink granules (H&E staining)
In circulation (blood): 4% of all WBC
Role in immune responses to parasites & allergies
Release granule content to kill (bigger) targets
Granules: eosinophilic
cationic protein,
peroxidase, MBP
Describe Mast cells
Located in tissues, close to blood vessels
Granules contain inflammatory mediators
Degranulation (e.g. histamine, serotonin) => inflammation
Main role: hypersensitivity type I (allergy), parasites
What are phagocytes?
Most abundant population of WBCs in blood (~70%)
- Early response (inflammation)
- Phagocytosis
- NETosis
- Killing of microbes by degranulation and release of toxic enzymes and compounds
Where are monocytes found?
Blood
Where are macrophages found?
Tissues
What do monocytes and macrophages secrete that leads to inflammation?
Cytokines
How are microbes recognized by the innate cells?
PAMPs: pathogen associated molecular patterns
Receptors on the surface and inside immune cells which can recognize (bind) a wide variety of molecules (patterns) from pathogens.
Each pattern recognition receptor (PPR) recognizes a distinct set of pathogen associated molecules.
PAMPs
Pathogen Associated Molecular Patterns
- Recognizes common microbial structures
- Structures shared by groups of related microbes
- Does not distinguish fine differences between pathogens
- Limited recognition (~1000 = 103 structures)
- Self/non-self discrimination: very good; never fails
Present on pathogens and not on host cells - Invariant structures: shared by entire class of pathogens (highly conserved)
Why are PAMPs essential for the survival of pathogens?
Prevents evasion of immune system by pathogens
e. g. ds viral RNA=> replication
e. g. lipolysaccharide => structure of bacterial membrane
What are PRRs?
Pattern recognition receptors (PRRs)
- Present on cells of innate immune system
- Recognize conserved molecules on pathogens
- Detect foreign invaders or aged/damaged host cells
- Toll like receptors (TLRs)
- C-type lectin receptors (CTLRs)
- NOD-like receptors (NLRs)
- RIG-like helicase receptors (RLRs)
- Scavenger receptors
Where are PRRs present?
Macrophages, eg TLRs
other cells too
What do TLR2 and TLR4 recognise?
TLR2: lipoproteins, peptidoglycans.
TLR4: LPS and Lipoteichoic acids
Other TLRs recognise bacterial DNA sequences (unmethylated CpG)
+ single/double stranded DNA
What do mannose and scavenger receptors recognise?
Glucan, mannose and scavenger receptors: sugars present on the surfaces of a large range of microorganisms.
Receptor recognition mechanism for TLR
TLR 2 recognises peptidoglycan
→ induces signal to produce transcription factor
→ acts on DNA inducing cytokine production
What is ‘eaten’ in phagocytosis?
Pathogens
Damaged cells
Dead cells
Nutrients
(innate immune system)
Why is phagocytosis necessary?
Protection from pathogens
Disposal of damaged/dying (apoptotic) cells
Processing and presentation of antigens (Ag)
Activation of adaptive immune system
What are the steps of phagocytosis?
- Chemotaxis (mobilisation to site of infection/injury)
- Recognition and attachment to microbe/dead cells
- Engulfment
- Killing of ingested microbe/dead cells
Phagocyte mobilisation: chemotaxis
Movement of cells towards site of infection
Guided by chemo attractants released by:
- bacteria (fMLP)
- inflammatory cells eg chemokines (IL-8)
- damaged tissues
Produce gradient of chemokines attracting other cells to injury site eg nutrients
Once a phagocyte is activated what enzyme is formed?
Assembly of phagocyte oxidase
- generation of superoxide anion
Oxygen dependant killing of pathogens
Oxidising radicals (ROS and NO) Kill phagocytosed microbes
Oxygen independent killing of pathogens
Secretion of lysosomes and antibacterial peptides:
- Proteolytic enzymes (cathepsins): degrade microbes
- Lysozyme: breaks bacterial walls
Lactoferrin: binds iron - not enough left for bacteria
- Defensins: destroy bacterial walls
How can encapsulated microorganisms be effectively phagocytosed?
OPSONISATION
via antibodies
Complement activation leads to…
Opsonisation of microbes -> increased phagocytosis
Inflammation
Lysis of microbes -> recruitment + activation of leucocytes
NK cells
Kill virus infected cells, malignantly transformed cells
Express cytotoxic enzymes (lyse target cells)
Dendritic cells
In skin, mucosa and tissues
Capture microbes + phagocytose
Eliminate pathogen + present antigen to T cells
links innate + adaptive
DCs shuttle antigen from peripheral tissue sites to lymph nodes where it can be ‘presented’ to T cells
Key points on adaptive immune system
- Triggered by exposure to microbes (acquired)
- Lag time (exposure => max response (days))
- Combats pathogens that evade/overwhelm IIS
- More efficient at eliminating infections
- Exquisite specificity
- Remembers pathogens (memory !)
- Better / faster with each repeated exposure
What kind of immunity are:
T cells
B cells
involved in?
T cells- cellular immunity
B cells- humoral immunity
Types of T cell
T helper (Th)
Cytotoxic (CTLs)
Regulatory (Treg)
Th (helper cells)
CD4
Activate macrophages
Help B cells to produce antibodies
Th1, Th2, Th17 cells
CTL (cytotoxic) T cells
CD8
Kill cells infected with microbes
Kill tumour cells
Treg (regulatory) cells
Inhibit function of other T cells and immune cells
Control of immune responses
What are the types of B lymphocytes and what do they do?
FOLLICULAR B CELLS
- spleen, lymph nodes
- produce high affinity IgG class/switched antibodies
- antibodies directed against protein antigens
MARGINAL ZONE B CELLS
- spleen, lymph nodes
- produce IgM class antibodies
- antibodies directed against polysaccharide, lipid antigens
B-1 CELLS
- peritoneal cavity, mucosal tissues
- produce natural low-affinity IgM class antibodies
- antibodies directed against polysaccharide, lipid antigens
REGULATORY B CELLS
- produce anti-inflammatory cytokines
How are antigens recognised in adaptive immune system?
- Antigen receptors on B cells and T cells
=> Ig on surface of B cells; TCR = T cell receptor - Can virtually recognize any microbial structure
- Not encoded in germline
- Distinguish
=> antigens of different microbes
=> antigens on same microbe - Self-non-self discrimination can fail- autoimmune diseases
Innate vs Adaptive immunity
CD8 vs CD4 cells
• CD8+ cells: Kill bacteria-infected cells – MHC-I antigen presentation; cytotoxic granules (perforin; granzymes);
• CD4+ cells:
– TH1: Activate macrophages by production of IFN-gamma, to aggressively ingest
antigen and to kill ingested bacteria.
– TH2: Stimulate B cells (via production of cytokines such as IL-4) to differentiate into
antibody-producing plasma cells
How to overcome antibody defences ?
• Secretory IgA proteases
• Immunoglobulin binding proteins - protein A
• Antigenic variation
• Capsules/LPS
– polysaccharides are poorly recognised antigens
Antibacterial roles of antibody
Types of immune response
How to overcome T cell mediated defences ?
- Superantigens - dysregulated T cell activation
- Toxins - leukocidins
• Cytokine disruption
– mimics and inhibitors
- cytokine proteases
- Antigenic variation
- Disruption of cell cycle and apoptosis control
Antigenic variation
Concept checks
Immune evasion
- Co-evolution of host and pathogen defence mechanisms
- Pathogens evade innate and adaptive immunity
- Pathogen overcomes natural barriers
- Pathogens signal danger
- Mucosal surfaces - sIgA proteases
- Avoid Complement - capsules and altering regulatory factors
- Blocks opsonisation and phagocytosis
- Intracellular survival - multiple mechanisms
- Cytokine disruption and control
- Antibodies – proteases, binding and antigenic variation
- Cell mediated immunity – antigen presentation and variation
What cells mediate humoral and cellular immunity?
B cells mediate humoral immunity
T cell mediate cellular immunity
Antibodies act by…
binding to extracellular microbes and toxins
- neutralise them (prevent Ags binding to receptors or cells, block Ag entry/effects)
- eliminate microbes
- opsonisation = ↑ phagocytosis
- complement activation = opsonisation, ADCC
T cells of cell mediated immunity
Th1 (CD4+): help phagocytes to kill ingested microbes
Th2 (CD4+): help eosinophils/mast cells to kill helminths
Th17 (CD4+): role in defense against bacteria & fungi
CTL (CD8+): kill cells infected by microbes that grow freely in the cytosol
What do T cells recognise?
T cells recognise cell bound Ags , peptides from Ags only bound to MHC
gamma delta T cells recognise antigens that are NOT peptides
Dendritic cells move antigens where?
From peripheral tissue sites to lymph nodes where it can be presented to T cells as APC
Where are naive T cells activated?
Secondary lymphoid organs- spleen and lymph nodes
Which cells express high levels of NHC class II?
Only activated professional APCs express high levels of MHC class II
These APCs also express co-stimulatory molecules
T cell receptor TCR- antigen receptor
alpha beta TCR in CD4+ and CD8+
gamma delta TCR in gamma delta T cells
Major histocompatibility complex (MHC)
Which MHC is on all nucleated cells?
MHC class I
class II on APCs
Structure of MHC
two chains; form a groove that holds a peptide to be presented to T cells
MHC I vs MHC II structyre
3 alpha chains (a1, a2, a3)
1 B2 microglobulin
peptide binding cleft
2 alpha chains (a1, a2)
2 beta chains (b1, b2)
peptide binding cleft
Antigen recognition by CD4+ T cells
1) pathogens living outside cells (eg bacteria)
- taken up by APCs vesicles via phagocytosis
- Ags processed in endo-lysosomes into peptides
- MHC II produced in ER- taken to endo lys
- processed peptides loaded onto MHC II
- peptide:MHC II complexes displayed on APC cell surface
- CD4+ helper T cell scan peptide:MHC II complexes on APCs
2) pathogens living inside cells cytosol (eg viruses)
- viral proteins produced/released in cytosol
- processed by proteosome into peptides
- processed peptides to ER -> loaded onto MHC I
- peptide:MHC I complexes displayed on cell surface
- CD8+ cytotoxic T cells scan peptide:MHC I complexes
Cytosolic vs Exogeneous Ags presentation
MHC II: exogeneous Ags
MHC I: cytosolic Ags
First signal in antigen recognition
Is the signal that initiates the immune response, so that the immune response is antigen-specific
TCR in T cell recognises the antigen in the context of MHC:
CD4 TCR recognises MHC II/peptide complex
CD8 TCR recognises MHC I/peptide complex
but needs more signalling- TCR signalling not enough
Second signal in immune response
TCR signaling is NOT enough to activate a naïve T cell
co-stimulatory molecules are also required:
1) B7:CD28
- CD28 is expressed by the T cell
- B7-1 (CD80) and B7-2 (CD86) molecules are expressed by the APC
2) ICOS: ICOS Ligand
- ICOS is expressed in T cells
- It binds to ICOS-L which is expressed in APCs
ICOS may play a role later in activation as not expressed on naïve T cells
3) CD40:CD40L
4) 41BB: 41BBL
5) OX40: OX40L
When is 2nd signal provided?
Only when APC is activated
How do T cells activate APCs?
T cells activate APCs via CD40 – CD40L interaction, enhancing T cell responses.
Upon activation, T cells upregulate CD40L, which binds to CD40 on DCs and stimulates the production of co-stimulatory molecules and cytokines by the DCs, thus enhancing T cell proliferation and differentiation.
What impact do negative costimulatory molecules have on the 2nd signal?
CTLA-4 and PD-1, LAG3
They inhibit the downstream effector processes initiated by TCR MHC/peptide interaction
Reduce inflammation after the infection has cleared
Not expressed by naïve T cells, there are induced upon activation
PD-1: Programmed cell death protein 1.
Mainly expressed in T cells in peripheral tissues.
Cytotoxic T-Lymphocyte Antigen 4: CTLA-4
CTLA-4 is expressed approx 2-3 days post stimulation
It has high affinity for CD80 but opposing effects to CD28.
It is mostly expressed in T cells in secondary lymphoid organs.
Peak levels of expression lower than CD28 but avidity of interaction is much higher
Therefore, competes favourably with CD28 for ligation to CD80/86
3rd signal
Cytokines from T cells
IL-2- for T cell proliferation
IL-2 is a growth, survival and differentiation factor for T cells and Tregs.
Signal 1 and 2 together initiate expression of cytokine genes (Signal 3):
IL-2 which is directly mitogenic and induces proliferation
Polarising cytokines which induce differentiation/ polarisation
What induces T cell polarisation into the different subsets?
The polarizing cytokines
These are generated by the stimulating APC
Which cytokines they produce depends on:
- The cellular origin of the APC
- The maturation and activation status of the APC
- Which pathogens or inflammatory mediators were encountered by the APC
- In which tissue environment the encounter takes place
Signal 3 and CD4 T cells
Various forms of signal 3 induce the differentiation of naive CD4 T cells down distinct effector pathways.
Each effector T cells expresses a master controller transcription factor
This transcription factor controls the expression of effector cytokines
Th1 mediated immune responses
Main cytokine: IFN- gamma
Main role: activate phagocytes (macrophages) -> more destruction of pathogen
also stimulate IgG Abs production -> more phagocytosis of microbes
Th1 mediated diseases: granulomas, autoimmunity
Th1 help signals
- . Contact-mediated signals: CD40L-CD40 ensure that only the infected macrophage will receive help from Th1 cells (specificity)
- Soluble signals: IFN-y
induces macrophage activation
Th1-mediated macrophage activation
↑ ROS, NO
↑ lysosomal enzymes => ↑ killing phagocytosed microbes
Secretion of cytokine: TNF-alpha, IL-1, IL-12
=> local inflammation: ↑neutrophils and monocytes recruitment
=> ↑phagocytosis
Th2 mediated immune responses
Responses against infections with helminths
- Too large to be phagocytosed by neutrophils & Macrophages
- Thick coat: resistant to microbicidal activities of PMN/Macrophages
Th2 cells help B cells produce Abs that opsonise helminths
Abs activate eosinophil/mast cell => destroy helminths
Th2 differentiation
TH2 differentiation occurs in response to phagocyte - independent immune responses.
TH2 polarizing cytokine is IL-4
- Dendritic cells do not make IL-4
- Eosinophils, basophils and mast cells produce IL-4. ILCs also produce IL-4
Transcription factors: IL-4 activates STAT6 which promotes expression of GATA 3
GATA 3 is a transcriptional activator of IL-4 and IL-13 genes
Th2 cytokines
IL-4, IL-5, IL-13
IL4 and 13- stimulate IgE production
IgE opsonises helmniths
Th2 eosinophils
Eosinophils have receptors that bind to Fc of IgE
IL-5 activates bound eosinophils => kill helminths
release granule content: MBP, MCP => can destroy tough integument of worms
=> Release: vasoactive amines, TNF-, lipid mediators
=> Local inflammation => helps destroy parasite
Th1 vs Th2 immune responses
CD8+ CTL mediated immune response
eliminate intracellular microbes in cytosol eg viruses
CTLs key in immunity to tumours and rejection of organ transplants
deliver kiss of death. sexy
then detach but target dies- release cytolytic proteins stored in secretory granules- trigger apoptosis in target cell
Cytolytic proteins
Cytoskeleton reorganisation; granule exocytosis
Perforin: forms pores => delivery of granzymes
Granzymes A, B, C: initiate apoptosis
Delivered at the site of contact between CTL:target
=> prevents killing of neighbouring healthy cells
