S1: White Blood Cells and Phagocytosis Flashcards
What are granular leucocytes also called?
Granulocytes
Describe the nucleus of granulocytes
Polymorphonuclear cells
Give examples of granulocytes
Neutrophils
Mast cells
Basophils
Eosinophils
What colour does basophils stain in H and E staining?
Blue granules
How many basophils are in circulation (rough percentage of WBC)?
0.1-0.2 %
What are the main roles of basophils?
Hypersensitivity: type I allergy
Degranulation (e.g. Histamine) for inflammation
Where are mast cells located?
In tissues close to blood vessels
Where are basophils and eosinophils located?
In blood circulation
What do granules in mast cells contain?
Inflammatory mediators
What do mast cells release for inflammation?
Degranulation(but less than basophils)
What is the main role of mast cells?
Hypersensitivity type I allergy
Parasites
What colour does eosinophils stain in H and E staining?
Pink granules
How many eosinophils are in circulation (rough percentage of WBC)?
4% of all WBC in circulation
What is the role of eosinophils?
They have a role in immune responses to parasites and allergies
Release granules content to kill (bigger) target
Give an example of a granule in eosinophils
Eosinophils cationic protein
Describe the nucleus of monocytes/macrophages
They are non granulated
Mononuclear
What is the main function of dendritic cells?
Antigen presenting cells
They capture microbes and are involved in phagocytosis but they then display the microbes antigens to T cells.
This links innnate and adaptive immune system
What is the definition of phagocytosis?
Cell eating microorganism/other cells/nutrients
What’s is the role of phagocytosis?
- protection from pathogens
- disposal of damaged/dying (apoptotic) cells
- processing and presentation of antigens
- link innate and adaptive immunity
What are phagocytes?
They are specialised cells that identify, ingest and destroy pathogens
Give some examples of phagocytes
Neutrophils
Macrophages
Dendritic cells
Describe the nucleus of neutrophils
They are polymorphonuclear leukocytes and they have multiple lobes on their nucleus
What is the lifespan of neutrophils in blood and tissue?
They have a relatively short lifespan:
- hours in blood
- days in tissue
Neutrophils are part of the first response to inflammation and are involved in the phagocytosis and killing of microbes
Name some enzymes that they use to do this and what are they contained in?
Lysozyme
Collagenase
Elastase
They are contained in granules
What is the difference between monocytes and macrophages?
Monocytes are in the blood and once they enter tissue they are called macrophages
Name the inflammatory factor that macrophages secrete
Cytokines
Where are dendritic cells found?
Skin, mucosa, tissues
Name the 4 steps in phagocytosis
- Phagocyte mobilisation (through chemotaxis)
- Recognition and attachment
- Engulfment
- Digestion and hence pathogen destruction
What is phagocyte mobilisation?
The movement of cells towards site of infection guided by chemoattractants
What releases chemoattractants?
- bacteria (fMLP)
- inflammatory cells (which release chemokines- IL-8)
- damaged tissue
What is recognition of pathogens and attachment in phagocytosis?
Recognition is to ensure phagocytes only react to invading foreign pathogens instead of body’s own tissues.
This uses ‘Pathogen Associated Molecular Patterns’ (PAMPs) on pathogens
To recognise PAMPs, phagocytes have Pattern recognition receptors (PRRs)
Opsonisation helps identify the pathogens for phagocytosis.
Phagocytes have receptors for these opsonins so they can attach
What are pathogen associated molecular patterns (PAMPs)?
They are present on pathogen and not host cells.
They are invariant structures (shared by an entire class of pathogens).
They are essential of the survival of pathogens but they also prevent pathogens from completely evading phagocytes
Give some examples of PAMPs
In gram positive bacteria, PAMPs would include lipoteichoic acid (constituent of cell wall and peptidoglycan
In gram negative bacteria, the lipopolysaccharide layer can act as a PAMP and the peptidoglycan
In mycobacteria, the peptidoglycan cell wall also acts as PAMP as well as other structures
What is the role of pattern recognition receptors (PRRs) and where are they found?
PRRs recognise PAMPs and therefore help detect foreign invaders.
They are present on phagocytes and other cells such as epithelial cells
List some types of pattern recognition receptors (PRRs)
- Toll like receptors (TLC) present in plasma membrane
- C type lectin in receptors (CTLRs)
- NOD like receptors (NLRs) as free proteins in cytoplasm
- RIG like helicase receptors (RLRs) which are cytosolic receptors for viral dsRNA (double stranded RNA)
- scavenger receptors which recognise various bacterial cell wall
Toll like receptors (TLRs) play an essential role in innate immunity
Name some PAMPs that human TLRs recognise:
- lipopolysaccharide (gram negative)
- lipoteichoic Acid (gram positive)
- bacterial DNA sequence
- single double stranded viral DNA
- glucans
What do TLRs also do apart from recognise PAMPs?
Stimulate production of inflammatory cytokines
What is the function of opsonisation?
To facilitate phagocytosis with the recognition of microbes
What is opsonisation?
It is the coating of microbes which targets them for phagocytosis.
The proteins that coat microbes are called opsonins.
This facilitates phagocytosis as phagocytes have receptors for these opsonins which makes phagocytosis quicker and more efficient
Give some examples of opsonins
Antibodies such as IgG
Proteins of complement system such as C3b, C4b
Why is opsonisation important for encapsulated microorganisms?
Encapsulated microorganisms can hide from PRRs so they require opsonisation for phagocytes to recognise them
What is engulfment?
Where the pathogen enters the phagocyte
This is done through phagosome formation and maturation
Describe phagosome formation and maturation
When the microbe is attached to the phagocyte, the actin skeleton begins to get rearranged which causes and controls membrane remodelling.
Pseudopods form around the sides of the microbe and eventually the microbe is completely surrounded.
It then buds into the cytoplasm in a phagosome.
The phagosome then fused with lysozyme to forms phagolysosome. The pathogen is then destroyed by the lysosomal enzymes.
What are the structures formed during membrane remodelling called?
Pseudopods
What is the vesicles containing microbes in the cytoplasm of a phagocyte called?
Phagosome
What is a phagolysosome?
A phagosome that fuses with a lysozyme
Describe oxygen-independent killing of pathogens
This utilises enzymes from the lysozyme
- proteolytic enzymes which degrade microbes
- lysozyme which breaks down bacterial cell walls
- lactoferrin which binds iron that bacteria needs
- defensins which destroy bacterial cell wall
Describe oxygen-dependent killing of pathogens
This first involves assembly of NADPH oxidase on the phagolysosome.
This generates superoxide anions (an oxidising radical) which break down bacteria in the activated phagocyte
How do oxidising radical kill pathogens?
Examples using Equations - *not an LO
O2——> O2- (superoxide)
H2O + O2- ————> H2O2 + OH-
(Enzyme dismutase)
Argenine + O2 ————> NO + citruline (NO is radical)
(iNOS - inducible NO synthase)
NO + H2O2 ————-> peroxynitrate radicals
List ways microbes evade phagocytosis
- Encapsulation so PRRs may not be able to detect PAMPs beneath it e.g. streptococcus pneumoniae
- certain microorganisms block engulfment e.g. yersinia
- block reactive oxidising radicals from destroying them e.g. salmonella is resistant to reactive oxidising species
- blocks fusion of phagosome and lysozyme so phagolysosome is not formed e.g. mycobacterium
- produce toxins which damage the membrane of phagocytes ultimately killing them e.g. staphylococcus aureus
What is apoptosis?
It is programmed cell death.
What cells destroy apoptotic cells?
Phagocytes quickly and efficiently remove the cells that have been programmed to die
Why is apoptosis known as ‘silent’ removal?
There is no inflammation
Explain apoptosis
This is done due to apoptotic cells expressing ‘eat me’ signals on their membrane. These are attracted to molecules on the phagocyte (which recognised that it needs to be eaten so engulfment takes place)
Viable cells express ‘don’t eat me’ signals on their surface which repulses phagocytes
What is secreted as a consequence of apoptotic cell removal?
‘Pro-healing’ cytokines which reduce inflammation and promote wound healing
What maintains self tolerance of the immune system?
Phagocytes which present the antigens if dead host cells to the immune system
