B-cells Flashcards
what is the Innate immune system?
→Rapid response
→Non-specific (generic anti-bacterial or anti-viral mechanisms)
→Most often fails to completely eliminate the infection
what is the Adaptive immune system?
→Delayed response →Highly specific →Usually eliminates infection →Memory →Long term immunity, but specific to that particular pathogen
what are the branches of adaptive immunity and what are they regulated by?
Humoral immunity
→Mediated by B-lymphocytes
Cellular immunity
→Mediated by CD8+ cytotoxic T- lymphocytes
→Both branches regulated by CD4+ helper T-lymphocytes (T-helper cells)
what is Humoral immunity?
→Humor = fluid
→Following an infection
→Plasma contains substances- “antibody (Ab)”
→which neutralise that specific infectious agent
→Demonstrate in vitro Or in vivo, e.g. treatment of rabies by infusion of antibody “adoptive immunotherapy
what is an antibody?
Protein- “immunoglobulin (Ig)”
→Migrates in the γ-globulin fraction on serum electrophoresis
→Each antibody binds to a specific antigen (most often a protein) on the infectious agent
→But plasma contains many different Abs
what is the structure of an antibody?
→Immunoglobulin protein
→Y-shaped
→Tetrameric
→2 identical heavy chains
→2 identical light chains
→Held together by non-covalent interactions
→and by –S-S- crosslinks between cysteine a.a. residues
→Each Ig molecule has two antigen binding sites
→flexible hinge region
describe light chains
→There are two types of light chain Kappa (κ) and lambda (λ)
→But any B-cell will only make one type
→Any Ig molecule will contain either kappa or lambda, never both.
→This phenomenon is called “light chain restriction”
how many regions do antibodies have and describe them
variable region
→Amino acid sequence varies from one Ig molecule to another
→Binds antigen at N- terminus
→constant region- C’terminus
→Responsible for effector functions E.g. activating complement, binding to phagocytes
what is Ig?
Ig is a glycoprotein (Carbohydrate added in the Golgi)
what happens if you treat Ig with protease?
Cuts molecule at hinge region
→Fab- fraction Antigen binding- It is composed of one constant and one variable domain of each of the heavy and the light chain
→Fc- fraction crystallisable- the tail region of an antibody that interacts with cell surface receptors called Fc receptors and some proteins of the complement system
what are the three ways in which antibodies fight infection?
COATING AND NEUTRALIZING
→if a virus is coated with Ab it cannot bind to its receptors
ACTIVATING THE COMPLEMENT
→which can blow holes in a bacterial cell membrane
OPSONIZATION
phagocytes have Fc receptors on their cell membrane
→bind to pathogens coated in Ab and phagocytose them
How does an Ab bind to an antigen?
→Non-covalent interactions →Electrostatic →hydrophobic →van der Waals forces →hydrogen bonds
→ Depends on the antibody binding site being exactly complementary, sterically and chemically, with a site on the surface of the antigen
→ The binding site on the antigen for one specific Ab is called an epitope
what are different types of B cells?
→The body generates over 100,000,000 different B-cells each making a different “random” Ig
→Each B-cell only makes one specific Ig
→These naïve B-cells sit around in lymph nodes doing not very much
what happens to the B cells in lymph nodes during an infection?
→During an infection, a small number of B-cells will, by chance, be making an Ig that binds one of the foreign antigens
→These B-cells are activated and begin to multiply- “clonal selection”
how does Lymphocyte Development happen in the Bone Marrow?
→ Haematopoietic stem cells differentiate into either
→common myeloid progenitor (neutrophils, red cells, platelets)
→ common lymphoid progenitor ( either pre-T or pre-B)
what are Primary and Secondary Lymphoid Organs?
→ HSC into Pre-B
→ imm B-cells
→ Imm B-cells into follicles containing resting B-cells
→ (secondary lymphoid organs, lymph nodes, spleen, gut etc)
how are B cells activated?
→Functional Ig is first expressed as IgM on the cell surface (sIgM)
→this acts as a “B-cell receptor” in a similar way to a growth factor receptor.
→The IgM does not have intrinsic tyrosine kinase activity, but associates with other tyrosine kinases
→Binding of antigen to IgM activates the tyrosine kinases and their signal transduction pathways
what does B cell activation require?
→Antigen binding to the B-cell receptor (sIgM), resulting in stimulation of signal transduction pathways
→Co-stimulation by T-cells
→The activated B-cell begins to secrete soluble IgM
what happens to activated B cells?
B cells activated
→Multiply rapidly
→Differentiate to become Ig secreting cells
→First make IgM
→Then undergo class switching to make Igs with the same Ag specificity but different heavy chain constant regions
what do memory B-cells do?
→Memory B-cells allow a very rapid response to a second exposure
→Immediate production of IgG rather than IgM
why are natural immune responses described as polyclonal?
→More than one clone of B-cells is generated
→More than one Ig is synthesised
→ Multiple antigens on organism
→Multiple epitopes on each antigen
→More than one Ig may recognise the same epitope
what is class (or isotype) Switching?
→The body can make different classes of Ig IgG, IgM, IgA, IgD, IgE differ slightly in heavy chain constant region amino acid sequence
→Have different functions note: there are actually 4 types of IgG (subclass IgG1 – IgG4) →And 2 types of IgA (subclass IgA1 and IgA2)
what are the heavy chain isotypes?
γ = IgG μ = IgM α = IgA δ = IgD ε = IgE
properties of IgM
→Always the first class of Ig made by B-cells during the primary response
→First made as a membrane bound protein on B-cell surface
→Activates B-cell by signal transduction
→ Later made in secreted form
→Activates complement
→Acts as opsonin
what does presence of IgM and IgG mean?
→Presence of specific IgM antibodies to an antigen indicates a recent primary response to that antigen
→Implies a current primary infection
→Presence of IgG antibodies may be due to past exposure to antigen
what is IgM structure?
→Membrane bound IgM is formed of a single Ig tetramer
→In secreted IgM five molecules of the basic Ig tetramer polymerise to form a pentamer
what is IgG and what does it do?
Major class of Ig in the circulation
→Very good at activating complement system
→Good as an opsonin
→Formed of a single Ig tetramer- monomer
what is IgA?
→Most abundant class in external secretions Milk, sweat, tears, gut secretions →Protects mucosal surfaces →Does not activate complement →Does bind Fc receptors triggering - Phagocytosis -Inflammatory reactions
what is IgA structure?
→In serum, occurs as a single Ig molecule In secretions, →most IgA is present as a dimer of two whole Ig molecules (+ accessory proteins)
what does IgE do?
→Physiological role in protection against parasitic worms →Binds to Fc receptors on mast cells and basophils →Triggers release of histamine BUT also involved in allergies!
→IgE produced in response to allergens (pollen, peanuts etc)
→Release of histamine causes symptoms of allergies Over response can cause anaphylactic shock
where is IgD found ?
→Extremely low concentration in circulation
→Also found on B-cell membrane- BCR
→Role is unknown
What are the different types of antibodies secreted?
They can be integral or secreted
Compare primary and secondary response
Primary
- smaller reponse
- more IgM than IgG
- Lower average antibody affinity, more variable
- Induced by All immunogens
Secondary
- larger
- increase in IgG, sometimes increase in IgA and IgE
- Higher average antibody affinity
- induced by mainly protein antigens
What is the difference between linear and conformational determinant?
Linear determinant- even if protein is folded or denatured it can bind to the antibody. The inaccessible part and the denatured can bind
Conformational determinant- requires protein to be folded in the right order
What is epitope or antigenic determinant?
– The region on the antigen where the specific antibody binds
Describe the variable region
• Within the variable region sequences are concentrated into certain segments called HYPERVARIABLE REGIONS, 3 in the VH and 3 in the VL
- Most variability lies in the HV3 domain
What is the difference between Fab and Fc antibody fragments?
Two Fab fragments (bind antigen) consisting of the light chain and two domains of the heavy chain (denoted VH and CH1)
- One Fc fragment (binds complement) consisting of the remainder of the heavy chain (CH2 and CH3).
Recall the cardinal features of the adaptive immune response(7)
Specificity
- diversity
- clonal selection and expansion
- specialisation
- contraction
- memory
- very low reactivity to self
What are the difference between marginal zone and follicular b cells?
Marginal zone B cells are noncirculating mature B cells that in humans segregate anatomically into the marginal zone (MZ) of the spleen and certain other types of lymphoid tissue. Have CD21
They produce T-dependent short lived high affinity plasma cells
- Follicular B cells populate the B cell follicles of the spleen and lymph nodes, associating with follicular dendritic cells.
Define cytokines
general term to describe various small proteins secreted by cells that serve to regulate the immune system. Can also have effects on other cells in the body.
Define chemokines
are a sub-type of immune molecules with involved in the movement and migration of immune cells
What is TNF-alpha?
Tumor Necrosis Factor (TNF-⍺).
Difference between B1 and B2 cells
B1: liver• Produce natural antibody
• present in the absence of infection
• They do not require T cell help producing mainly Igm short lived IgM plasma cells
B2:bone marrow. located in secondary lymphoid organs but they circulate. Produce the majority of high affinity antibodies and require T cell help. In the spleen and serve as a first line of defence against blood-borne pathogens, may or may not require T cell help
Describe the phases of the humoral response
- Native B cell recognises antigen and is activated by Helper T cells and other stimuli.
- Proliferation and differentiation into plasma cells, IgG expressing B cells, and high affinity Ig expressing B cells that turn into memory cells
Ig gene arrangement
- Variable region can be produced by one fragment that can be copied with D+J fragments forming one molecule
- Later the transcript is spliced and translated to heavy chain with Fragment and J fragment
- Cells also add nucleotide at exon/intron junction
Recall the different cytokines that initiate IgM+ switching
- Release of Igs ( into the circulation or onto mucosal surfaces)
- If it receives Tcell or cytokines, the B cell can switch its IgG
- If IgM+ receives cytokines IFN-gamma and IL-4 then it switches to IgG subclasses and IgE,
- If it receives mucosal tissues cytokines, TGF-B or BAFF then IgA
Describe the life cycle of a B-cell
- B-cell precursor re-arranges its immunoglobin genes in bone marrow
- Immature B-cell bound to self-surface antigen is removed from repertoire- negative bone marrow
- Mature B-cell bound to foreign antigen activated- migration to peripheral lymphoid organs and activation
- Give rise to plasma cells - secretion of memory cells in bone marrow and lymphoid
