Antibodies Flashcards
What are antibodies produced by?
B lymphocytes
What is the origin of B lymphocytes?
Lymphocytes arise from stem cells in bone marrow, and differentiate in the central lymphoid organs (red), B cells in bone marrow, T cells in thymus. They migrate in bloodstream to peripheral secondary lymphoid organs (yellow), lymph nodes, spleen, gut-associated lymphoid tissue (GALT), Peyers patches, tonsils, appendix.
What do B cells do before they are needed?
- Naive B cells, ie that have not met antigen, circulate from blood into peripheral lymphoid tissues.
- Peripheral lymphoid tissues are the main site of antigen encounter
What role does the lymphatic system play in moving B cells around the body?
• Lymphatics drain tissues of fluid, through the lymph nodes and into the thoracic duct. Recirculating B cells enter back into the bloodstream by the thoracic duct.
How are lymph nodes organised?
Organised into cortex and inner medula. Cortex has an outer section of B cells organised into follicles (yellow), and a paracortical area of T cells and dendritic cells (blue). Germinal Centres of B cell proliferation form during an immune response. Medulla consists of macrophages and antibody secreting B cells (Plasma cells). Antigen (in DC) enter through afferent lymphatics.
What are the two lymphatic/haemolytic areas of the spleen?
Red pulp (site of RBC destruction)
White pulp (lymphoid)
Describe blood flow in relation to B cell movement through the spleen
Blood carrying lymphocytes
and antigen enter from a trabecular artery into a central arteriole. They can then pass into a marginal sinus and exit through a trabecular vein.
What do B cells and antigens encounter when in the spleen?
The marginal sinus is surrounded by lymphocytes, and within it is the periarteriolar sheath (PALS), made up of T cells. B cell follicles and a B cell corona also form. Important - antigen enters from blood rather than lymph.
Describe the course of antibody production upon meeting an antigen
There are two responses; primary and secondary
In the primary response there is a lag phase while B cells activate, then antibody levels will begin to rise along a middling gradient as B cells begin antibody production peaking at around 12 days after the first vaccine.
In the secondary response antibody levels peak at over double the peak of the primary response in about 4 days, much faster than primary response.
What are B cells form a part of?
The adaptive immune response, the key features of which are antigen specificity, and memory.
What are the basic features of antibodies (Ab)
- Antibodies can be expressed as membrane bound (B cell receptor - BCR) or secreted forms.
- B cells express a single Ab specificity only.
- Ab have two separate functions, firstly to bind the pathogen that elicited its production, and secondly to recruit other cells and molecules that will lead to clearance or destruction of the pathogen.
- Ab are made of four polypeptide chains, two identical heavy (H) chains and two identical light chains (L). The H chains are disulfide bonded to each other, and each H chain is also disulfide bonded to a L chain.
- Two L chain exist, lambda and kappa. Any individual Ab has either lambda or kappa, never a mixture of each. In human the ratio is 2:1 in favour of kappa.
Describe antibody structure
There are 4 heavy chains and 2 light chains, arranged into 3 termini: two N termini of one light and one heavy chain each and one C termini made up of two heavy chains (makes a “Y” shape)
The different termini are bound together by disulphide bonds that form a hinge region that also contains a carbohydrate on the C termini aspect.
There are also three distinct regions defined by their variability. Two variable regions on the most distal half of each N termini and the rest being defined as the constant region
Where are the antigen binding sites on an antibody?
On the variable region found on each N termini
The top two prongs of the “Y”
What is the structure of the binding site of the antibody
The binding site for antigen
is contained in the Fab region, whilst The Fc region performs many of the functions of Ab, interacting with receptors etc. Fab and F(ab)2 fragments
are very useful tools in the laboratory, ie can be used to label cells without inducing the effects
of the Fc region, or in the case of Fab fragments, inducing receptor triggering or internalisation by crosslinking.
Describe hypervariability in V domains of antibodies
Three hypervariable loops determine antigen specificity by forming a surface complimentary to the antigen. Final specificity is determined by a combination of loops from H and L chains, and not either alone.
What are the three hypervariable loops found in antibodies?
HV3 (CDR3)
HV1 (CDR1)
HV2 (CDR2)
Describe antigen binding to Ab
Antigens can bind in pockets or grooves, or on extended surfaces. A) small peptide bound in pocket. B) extended peptide from HIV protein bound in groove.
C) extended surface interaction of Ab with hen egg-white lysozyme.
