1. An Overview of Immunology Flashcards Preview

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What is an antigen?

Anything the immune system responds to
Usually protein
Not necessarily 'bad'


What does an antigen receptor do?

It recognises the antigen
This is the fundamental basis of immunity
The basis of division into innate and adaptive immunity


What divides the immune system into innate and adaptive?

Antigen receptors
Germline-encoded pattern-recognition receptors in innate
Antigen-specific T and B cell receptors in adaptive


What does the binding of an antigen to antigen binding receptor do?

It leads to an effector mechanism which is an action to respond to the antigen


What is the main role of the immune system?

To protect from infection


Cells of myeloid lineage

Dendritic cells
Basophils (mast cells in tissue)



Myeloid lineage
Large cell with granular cytoplasm and lots of nuclei
Function: phagocytosis



Myeloid lineage
Cloudy reddish pink cell
Function: ?helminth infections



Myeloid lineage
Monocytes circulating, macrophages in tissue
big weirdly shaped granular cell
Function: phagocytosis and antigen presentation


Dendritic cell

Myeloid lineage
Lots of dendritic processes
Function: antigen presentation



Basophil in blood, mast cell in tissue
Myeloid lineage
Granular (tomato wearing sunglasses)
Function: ?helminth infections?


Lymphoid lineage

Similar size to RBC
Little cytoplasm with few granules

B cells: make antibody, antigen presentation
T cells
CD4: help other components of immunity
CD8: kill infected cells

T cells and B cells=ADAPTIVE IMMUNITY

NK cells: actually INNATE lymphocytes
Direct lysis of infected cells and antibody-dependent cellular cytotoxicity


NK cells

Lymphoid lineage
actually INNATE lymphocytes
Direct lysis of infected cells and antibody-dependent cellular cytotoxicity


Intercellular communication

Communication between cells via soluble mediators

Endocrine - via blood
Paracrine - messengers act on an adjacent cell
Autocrine - acting on itself
Juxtacrine - membrane-bound signalling molecule

Will often get more than one happening together in order to get a response


Communication: cytokines & chemokines

Cytokines are small proteins released by cells that have an effect on another cell
-important for communication between cells of the immune system and between immune system cells and other cells and tissues

Chemokines are similarly defined, but different structure, receptors and nomenclature
-main role is temporal and spatial organisation of cells and tissues


Main role of cytokines

communication between cells of the immune system and between immune system cells and other cells and tissues


Main role of chemokines

temporal and spatial organisation of cells and tissues


Antigen receptors

The receptor that cells use to recognise antigen is a key concept in immunology, and forms the basis of separating two immunological arms: innate and adaptive


Key features of innate antigen receptors

Do not recognise antigen specifically
Pattern recognition receptors’ (PRRs)
Recognise ‘pathogen associated molecular patterns’ (PAMPS)
Not clonally distributed


Mannose binding ligand

MBL binds with high affinity to mannose and fucose residues with correct spacing
Mannose and fucose residues that have different spacing are not bound by mannose binding ligand
Essentially evolved to recognise foreign things e.g. fungi, but not specifically fungi


Classical features of innate immune receptors/defences

Work quickly – first line of defence
Adaptive immunity takes more time to be activated
Unable to ‘learn’, as germline encoded and therefore cannot change – therefore no memory


Key features of adaptive antigen receptors

Recognise antigen specifically
T cell receptor, B cell receptor (antibody)
Produced by random somatic recombination events between gene segments
Huge receptor diversity
Clonally distributed
Permit specificity and memory in immunity


B cells

B cell receptor (antibody) may be surface-bound or secreted
Recognises intact antigen


The T cell receptor

T cell receptor is very similar to the B cell receptor

Only a surface receptor on CD4 and CD8 T cells

Recognises processed antigen in the form of linear peptides


Generation of adaptive immune receptor by somatic recombination events

T and B cell receptors are produced by random recombination events between V, (D) and J gene segments, producing a huge receptor diversity despite a small number of genes. The most useful receptors are selected after birth upon exposure to pathogens


How do T cells recognise antigens?

T cell receptors recognise peptides - short linear sequences of amino acids 12-18aa long, but only recognise this antigen when presented to them on a major histocompatibility complex (MHC)


Antigen processing pathways

Class I - CD8 - recognise virally infected cells, which will have foreign protein in cytoplasm, some of which goes through proteasome to be degraded into peptides to be presented on class I MHC proteins

CD4 pathway uses MHC II, takes up antigen and presents it on surface on MHC-II. uses phagolysosome not proteasome


What is clonal selection?

Cell gets selected out of many different cells depending on its ability to bind an antigen


What happens after primary infection?

After primary infection, most clonally-expanded T and B lymphocytes die off; a few remain as long-lived memory cells


Some examples of effector mechanisms

Barriers (skin, acid pH in gut etc etc)
Phagocytosis (enhanced by opsonisation)
Cytotoxicity (CD8 T cell, NK cell)
Antibody-dependent cellular cytotoxicity
Mast cell and eosinophil degranulation

Effector mechanisms are shared between innate and adaptive immunity. Adaptive immunity is defined by its receptors not by its effector mechanisms