The Adaptive Immune System Flashcards
What are the three main features of the adaptive immune system?
It responds to a specific pathogen
It has a ‘memory’ of specific antigens
It improves upon re-exposure
What is an antigen?
A molecule that provokes an immune response
What does the adaptive system require to become activated?
It requires the innate system
What are the cells of the adaptive immune system?
Lymphocytes
What are the two different types of lymphocytes?
B and T
How does the adaptive system response differ upon re-exposure?
It will be more rapid and more intense
Where do both types of lymphocytes originate from?
From progenitors in the bone marrow
Their precursor cell is the common lymphoid precursor
Where do B-cells mature?
In the bone marrow
Where do T-cells mature?
In the thymus
What does maturing mean in terms of lymphocytes?
2
Forming of a receptor
Gaining the ability to recognise and respond to specific antigens
What are cells called before they have matured?
Immature
What percentage of cells that go to the thymus die?
98%
What is it called when cells audition to become T cells?
T-cell education
What happens if a cell fails to become a T cell?
It is killed
What happens if a cell succeeds in becoming a T cell?
It enters the medulla
How many copies of its receptor does each lymphocyte have?
about 104 to 106 receptors
What is the receptor on a T cell called?
a T-cell receptor
What is the receptor on a B cell called?
a B cell receptor
What is present as the receptor on B cell?
antibodies
What are the two types of T cells?
Killer T cells
Helper T cells
What is the ratio between helper T cells and killer T cell?
You have twice as many helpers as you do killers
How are T cell receptors made?
They are made by a totally random process of gene shuffling
How do B cell receptors work?
2
The antibody molecule can be reproduced many times
These antibodies can be released from the cell at a distance
What are lymphocytes called after they have gotten their receptors?
Naive cells
What happens to lymphocytes once they have become naive cells?
They travel to the lymph nodes, spleen or other lymphoid organs to await activation
Describe the structure of the T-cell receptor.
4
Made of 2 polypeptide chains
Usually an alpha and a beta chain
Rarely a gamma and theta chain
Has variable regions that dictate what it will bind to
Describe the structure of an antibody.
Made of 4 protein chains
What type of cell secretes antibodies?
Plasma cells
What is the process by which clonal lymphocytes form?
Clonal expansion
What is clonal expansion?
The process by which a lymphocyte with the correct receptor replicates to form clonal lymphocytes
How do T cells become activated?
T cells need antigen presenting cells to show them their target
Name the three antigen presenting cells.
Dendritic cells
Macrophages/monocytes
B-cells
How does antigen presentation work?
4
The antigen presenting cells phagocytose pathogens
The pathogens are killed and chopped up into peptides
The antigen presenting cell display peptide fragments of the pathogens on their surfaces
The antigen presenting cell then approaches the T cells and if its receptor binds to the peptide the T cell will become activated
What does MHC stand for?
Major Histocompatibility Complex
Why do T cells need MHC molecules?
antigen presenting cells need to present their pathogen’s peptide in an MHC molecule to the T cells
Why are T cells known as MHC restricted cells?
Because T cells can only recognise antigens in the context of MHC
Where does antigen presentation and T-cell activation usually occur?
In peripheral lymphoid organs such as lymph nodes
What happens when an APC takes up a pathogem?
It becomes activated and migrates to a regional lymph node
What is the main function of lymph nodes?
They act as organising centres for immune responses
How does an APC know it has made contact with a pathogen?
3
APCs have pattern recognition receptors on their surface
These are called Toll Like Receptors (TLRs)
TLRs recognise specific pathogen associated molecular structures (PAMS)
What are the receptors on APCs called?
Toll Like Receptors
What does PAMS stand for?
Pathogen associated molecular structures (PAMS)
What does TLR stand for?
Toll like receptors
Give some examples of PAMS.
LPS
Double stranded RNA
Flagellin
What happens to an antigen presenting cell when a toll like receptor is activated?
(2)
The APC becomes activated
This allows it to migrate to a lymph node and interact with T cells
Is antigen presentation needed for memory cells?
Yes
What are killer cells also called?
Cytotoxic cells
What do killer T cells do?
3
They kill virus infected cells
They kill tumour cells
Apoptosis
What do helper T cells do?
2
They secrete cytokines to react with other cells
They help B cells make antibodies
What CD molecules are always expressed by T cells?
CD3
What CD molecules are always expressed by helper T cells?
CD4
What CD molecules are always expressed by killer T cells?
CD8
What type of cell is needed to activate a killer T cell?
Any nucleated cell
MHC class 1
What type of cell is needed to activate a helper T cell?
APC
MHC class 2
What is an epitope?
A specific amino acid sequence from an antigen
Can B cells activate without T cell help?
Yes
But their function is limited
How exactly does a B cell activate without the help of a T cell?
(5)
If more than one antibody reacts with the epitopes on an antigen they will become cross linked
This cross linking activates the B cell
B cell then becomes a plasma cells
Plasma cell can now secrete antibodies
However, it will only make IgM antibodies (1/5 types)
Explain how a helper T cell activates a B cell
4
A B cell has a peptide (its an APC) and its trying to find a T cell
If a Helper T cell recognise the peptide it can provide cytokines to the B cell
These cytokines help the B cell make antibodies
This also allows memory B cells to be made
What are antibodies?
2
Soluble versions of the B cell receptor
Immune proteins synthesized and released by plasma cells
What are antibodies also called?
2
Gamma globulins
Immunoglobulins (Igs)
Describe the structure of an antibody.
4
4 protein chains - 2 identical light chains and 2 identical heavy chains
Chains held together by disulphide bridges
2 regions - fab region (fragment with antigen binding) and the Fc region (fragment that is crystallisable)
Two binding sites for antigens
What are the two regions in an antibody?
Fab region
Fc region
What is the fab region of an antibody?
Fragment with antigen binding
What is the Fc region of an antibody?
Fragment that is crystallisable
How many different types of antibodies are there?
Five
What are the five different types of antibodies?
IgM
IgA
IgD
IgG
IgE
What does each variable region of an antibody contain?
An antigen-binding site
What are the four main ways antibodies deal with antigens?
Precipitation/agglutination
Lysis
Neutralization
Opsonisation
Explain precipitation
Occurs when antibodies bind their soluble targets into clumps
Explain agglutination.
Occurs when antibodies cause clumping of insoluble particles
Something insoluble is bound up by antibodies
What is the function of precipitation/aglutination?
This sequesters the target and increases the likelihood of phagocytosis
Something soluble is bound up by antibodies
Explain how antibodies use lysis.
5
Antibodies bind to a bacterium
The complement system is activated
Complement proteins form a MAC
This perforates the bacterial cell membrane
Cell lyses
Explain how antibodies are involved in neutralisation of antigens.
(3)
Antibodies bind to their target and interfere with the pathogen function
Antibody interferes with viral entry into host cell
Antibody binds to toxins and renders them inactive
How are antibodies involved in opsonisation?
2
Antibodies bind to pathogens
This marks out pathogens for phagocytes
How long is the lag period after the initial encounter until antibody production?
It takes 5-7 days
What happens ten days after an initial encounter?
Plasma antibody levels peak and begin to decline
How is the memory response greater than the initial response?
(4)
A shorter lag time-
Plasma cells that last longer-
Higher Antibody levels achieved more quickly-
Higher efficiency of binding between Antigens & Antibodies
