Initiation of Acquired Immune Responses (B cells and Antibodies) Flashcards
Revision (36 cards)
What are the two types of lymphocytes?
T cells and B cells
What is the role of B cells?
Mature in the bone marrow (Primary lymphoid tissue).
Responsible for humoral immune responses.
“humors” = body fluids
Produce antibodies that attack pathogens circulating in the blood and lymph.
Key role in defence against extracellular (and intracellular) pathogens.
What happens if B cells and T cells react to self-antigens?
As they develop, B cells and T cells learn to distinguish self form non-self.
If they react to self-antigens they are (usually) destroyed or inactivated.
What is the role of T cells?
Mature in the thymus (Primary lymphoid tissue).
Responsible for cellular immune responses.
Key role in defence against intracellular pathogens
- CD4+ T cells which are key regulators of the entire immune system.
- CD8+ T cells kill virally infected body cells.
What are antibodies and what are their roles?
An antibody is a protein that is produced in response to a particular molecule (antigen), and has the property of binding specifically to tat antigen.
Antibodies are also known as immunoglobins.
Antibodies are proteins that bind to one specific antigen.
They are a complex of 4 polypeptide chains (2xlight chain + 2x Heavy chain).
Each antibody as unique variable region that binds to one specific antigen.
Millions of different antibodies are present I our bodies - able to respond to millions of different antigens.
Antibodies exist in membrane bound and soluble forms.
Different classes of antibodies exist
- IgM
- IgG
- IgA
- IgE
- IgD
What are Antigens and what are their roles in the body?
An antigen is any substance which can cause an adaptive immune response by activating B cells and T cells.
Adaptive immune responses are induced by specific structures called antigens.
Individual cells/pathogens can contain many different antigens
- Individual antigens can contain many different antigenic epitopes.
- Each specific antibody can bind to only one specific antigenic epitope.
How do B cells use antibodies and antigens?
B cells use membrane-bound antibodies as a receptor to recognise and bind to antigens.
- Different B cells express different antibodies.
- One B cell expresses ~50000 copies of one specific antibody.
In what way does the adaptive immune system have specificity and how is each response unique to an individual pathogen?
individual B cells and T cell express only one type of antigen receptor on their cell surface - so each individual cell can only respond to one specific antigen.
There are a limited number of each antigen-specific cell type within the human body (space) - these few antigen-cells are not sufficient to kill and eliminate an antigen-carrying pathogen.
How are PAMPs recognized by non-specific recognition?
There are a limited number of PAMPs, which are common to many different pathogens.
Only a small number of different PRRs are required to stimulate innate immune cells.
Common structures (PAMPs) recognized by innate leukocytes.
How are antigens recognized by specific recognition?
Millions of different antigens - unique to individual pathogenic species.
Individual T cells and B cells only express one specific antigen receptor, which binds to only one specific antigenic epitope.
Unique structures (antigens) recognized by adaptive leukocytes.
Adaptive immune responses occur in secondary lymphoid tissues. Where do the T cells and B cells develop?
Antigen-specific cells and B cells develop in primary lymphoid tissues
- Bone marrow
- Spleen
Lymphocytes develop initially in the bone marrow and then mature in either the bone marrow (B cells) or in the thymus (T cells).
Where do B cells and T cells circulate within the body?
Mature, quiescent, antigen-specific T cells and B cells constantly re-circulate between different the blood, secondary lymphoid tissues and lymphatic vessels.
What state do lymphocytes remain in, where do they meet their antigen and become activated?
Lymphocytes are inactive until they meet their specific partner antigen within a secondary lymphoid tissue.
Secondary lymphoid tissues are sites where T cells and B cells are activated by antigens
Lymph nodes - important stations for monitoring tissues infections.
Spleen - important in monitoring blood-borne infections.
Mucosal - associated lymphoid tissues - important in monitoring throat/gastrointestinal tract infections.
What happens to B and T cells if they don’t encounter a specific antigen?
After several days, if they don’t encounter specific antigen, B cells and T cells return to the blood system via the efferent lymphatics.
Naïve T cells and B cells use the process of Transendothelial Migration to enter lymph nodes from High Endothelial Venules (HEV). What is this process?
Lymph nodes are key sites of pathogen detection and are therefore organized into distinct functional areas to facilitate efficient activation of B cells and T cells.
- Lymph flows into lymph nodes through the afferent lymphatic vessels.
- B cells and T cells enter lymph nodes through high endothelial venules (HEV).
- Once B cells come through the HEV, they immediately go into the lymphoid follicle.
- Once T cells come through the HEV, they stay in the -cell area of the lymph node and interact with dendritic cells tat come in through the afferent lymph.
- Both lymph and lymphocytes leave the lymph node by first going through the medullary sinus and then out through efferent lymphatic vessels. Efferent lymph eventually flows back into blood circulation via the subclavian vein.
What is the acquired immune response mediated by B cells (aka: the Humoral Immune response) and how does this process work?
B cells produce specific antibodies in response to specific antibodies in response to specific antigens.
Membrane bound, antigen-specific IgM (or IgD) is used to bind to their target antigens.
Once activated, these B cells clonally proliferate and differentiate into 2 different types of effector cells:
Plasma cells
- Produce and secrete soluble, antigen-specific antibodies
Memory B cells
- Long lived cells that continue to circulate around the body.
How do B cells encounter antigens?
Specialized cells within B cell zones can “trap” opsonised antigens - these cells express opsonin receptors.
What happens after B cells encounter antigens?
B cells need to receive 2 signals to become fully activated and clonally proliferate in response to protein antigens:
- Antigen
- “Helping” signals
After receiving both signals, antigen-activated B cells clonally proliferate (i.e. enter the cell cycle and divide many times over)
- The resulting daughter cells all express BCRs that have the same antigen-specificity as the parent B cell.
- These highly proliferative cells form a secondary follicle within the B cell zone (known as the Germinal Centre).
What are the 2 types of signals in protein antigens?
- BCR binding to antigen
2. Help from TH(H is subscript) cells
What are the 2 types of signals in other types of antigens?
- BCR + antigen
2. PRR + PAMP
What are the 2 types of signals in antigens with repetitive antigenic epitopes?
Signal 1+ 2: Multiple BCRs + antigens engaged
The activated B cells once activated, differentiate into plasma cells, how does this occur?
This expanded population of cells now has sufficient numbers to be able to dal with any pathogen tat expresses the specific antigen on their surface
- They stop proliferating and differentiate into antibody-secreting plasma cells.
- These secreted antibodies recognize the same specific antigen as the original parent B cell.
How do antibodies help kill and eliminate antigens (i.e. pathogens)?
Antibodies can bind to soluble antigens o to membrane-bound antigens expressed on the surface of microbes or other immune cells.
Antibodies provide immunological defence against extracellular and intracellular microbes (most bacteria, viruses)
- Recognise the microbe as foreign
- Inactive/eliminate the microbe
Recognition Function
- Binding to antigen mediated by variable region sites
Effector function
- Clearance mechanisms mediate interaction of the Heavy chain constant region with effector molecules
- Complement
- Fc receptors
What is IgM and what is it’s role?
IgM: present only in plasma and secretions (too large to enter tissues)
In membrane-bound form serves as the B cell antigen receptor (monomer)
- Functions: B cell activation
IgM (in its secreted form is the first Ig type produced during a humoral (adaptive) immune response.
IgM present in plasma and secretory fluids as a pentamer.
Functions of IgM:
- Agglutination (immune complex formation)
- Complement system activation