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An Introduction to the Immune System
CLASSES OF PATHOGENS
- The immune system protects against four classes of pathogen: bacteria
(e
...
salmonella enteritidis/food poisoning; mycobacterium tuberculosis);
viruses (e
...
variola/smallpox; influenza/flu; HIV/AIDS); fungi (e
...

ringworm/flush); parasites (e
...
protozoa; worms)
...

- Therefore, immune systems need different ways to recognise and clear
different pathogens
...

CARDINAL FEATURES
- Immune system needs: specificity (so only recognises foreign cells),
immunological memory (so it responds faster) and self-discrimination
(so that the immune system doesn’t attack self-tissue)
...

- Innate immune system – generic, rapid, helps guide the adaptive
immune system
- Adaptive immune system – acts after 96 hours but is quicker after the
first infection
- Note: skin is difficult for pathogens to penetrate whereas mucosal
surfaces are more delicate, and therefore easier to penetrate
...

- Or they can enter via external epithelia such as: an external surface
(physical contact); wounds and abrasions (minor skin abrasions,
puncture wounds, handling infected animals); insect bites
...
Bacterial cell surface induces cleavage and activation of complement
...
One complement fragment covalently bonds to the bacterium, the
other attracts an effector cell
...
The complement receptor on the effector cell binds to the
complement fragment on the bacteria
...
The effector cell (phagosome) engulfs the bacteria, kills it and breaks
it down
...

When PAMPs are recognised by soluble PRRs, it leads to: direct attack of
microorganism by soluble PRR molecules; enhancement of phagocytosis
of PRR-bound PAMPs and a proteolytic cascade resulting in lysis of the
microorganism
...


PRRS INCLUDE
- Toll-like receptors (TLRs); C-type lectin receptors (CT LRs); NOD-like
receptors (NLRs); RIG-like helicase receptors (RLRs); scavenger receptors
...
e
...

- TLR engagement promotes NFκB-dependent transcription so molecules
like cytokines and chemokines can be produced by the cell to recruit
more cells to site of infection
...

- Macrophages formed from monocytes circulating in the blood
...

- Macrophages found all over the body where pathogens are likely to
enter – they are the first line of defence
...

PHAGOCYTOSIS AND KILLING OF PATHOGEN

COMPLEMENT AND MAST CELLS
- Complement is important in terms of tagging or opsonising pathogen so
it gets seen by effector cells (macrophages and neutrophils)
...

- Complement also causes rapid inflammatory response by generating
C3A and C5A molecules which bind to mast cells and cause them to
degranulate
...

- There are 3 different pathways that can activate complement: the
classical pathway (antibody dependent); the lectin pathway and the
alternate pathway (which are both antibody independent)
...


INNATE IMMUNE RESPONSE AT SITES OF INFECTION

ADAPTIVE IMMUNE RESPONSES AND SITE OF INFECTION AND BEYOND
- Sometimes innate immune system is not enough so the adaptive
immune system is needed to completely remove the pathogen
...

- Takes a long time because information about the infection needs to
travel all the way to the local lymphoid organs to activate them
...
If these were just
circulating around the body, there’d be no chance of the right T-cell or Bcell finding the right pathogen
...

- Information from infection site is taken to the local lymphoid organs via
antigen presenting cells (such as dendritic cells and macrophages)
...

- T-cells require antigens to be processed and presented to them but B
cells recognise the antigen in its native form
...

- Inflammatory response by mast cells causes vasodilation and increased
vascular permeability
...


ADAPTIVE IMMUNITY HAS TWO “ARMS”
- 1st is cell mediated immunity, which is driven by T-cells, where smaller
pathogens like viruses and bacteria are engulfed
...


INNATE/ADAPTIVE IMMUNITY

- Recognition mechanisms of innate immunity: rapid response (hours);
fixed; limited number of specificities; constant during response
...

- Both lead to common effector mechanisms for the destruction of
pathogens (engulfment, digestion, lytic enzymes)
...
Epithelial barrier
2
...

3
...

4
...

FOLLOWING PRIMARY INFECTION
- Pre-formed antibodies and effector T-cells in tissues and blood available
immediately
...

SUMMARY OF INNATE IMMUNE LOCAL RESPONSE
1
...

2
...

3
...

SUMMARY OF ADAPTIVE RESPONSE
1
...

2
...

3
...
Also remember, a single proginator cell gives rise to a large
number of lymphocytes, each with a different specificity
...
Then there is the
proliferation and differentiation of active specific lymphocytes to form a
clone of effector cells
...

- Intracellular vesicular infection: helper T-cells/ Th1 (activation by
cytokines)
...

IN SUMMARY…
INNATE IMMUNITY (macrophages, granulocytes, mast cells, NK cells,
complement proteins, cytokines):
- Generic defence against categories of microbes

- Recognises common structures/characteristics of microbes and
changes to cells and tissues brought about by infection
- Inherited recognition repertoire
- Quickly activated

-

ADAPTIVE IMMUNITY (T and B lymphocytes, antibodies, cytokines)
Specific recognition of individual types of microbes
Recognises specific chemical structures of microbial molecules
(antigens)
Somatically generated recognition repertoire
Slowly activated (faster on 2nd exposure) and very efficient



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