B cells Flashcards
What are B cells?
Lymphocytes that develop in the bone marrow
What do B cells express?
Unique antigel receptor (B cell receptor, BCR)
What do B cells secrete?
Antibody (same specificity as BCR)
What are plasma cells?
Activated B cells that secrete antibody
What do memory B cells do?
Provide memory
What happens in the primary lymphoid organs?
Bone marrow development of B cells
What happens in the secondary lymphoid organs?
Lymph nodes, spleen, activation of B cells
Describe a B cell
B cells have BCR’s which are bound to the membrane and once secreted, BCR act as an antibody
What is the structure of a BCR?
@ heavy chains which stick into the cell and 2 light chains which are involved in antigen recognition
What is the structure of antibody?
Same as BCR except secreted not membrane bound
What is the surface of each B cell covered with?
Approximately 100,000 BCR which are mainly IgM and IgD antibodies
What does the BCR do?
Bind the antigen directly and activates the B cell
What does the activated B cell do?
Become a plasma cell and secretes antibody
What are the functions of an antibody?
Neutralisation, Opsonisation and Complement
What is neutralisation usually for?
Viruses
What happens in neutralisation?
Antibodies bind to the antigens on the virus (all over) coating it so that the virus can’t bind to receptors and enter the cell
What happens in opsonisation?
Antibody binds to the antigen and when bacteria is covered the phagocytes can recognise the bacterium as needing to be destroyed much easier
What happens in activation of complement?
Antibodies bind to the surface of the foreign cell and provide a site for the complement proteins to bind to and become activated
What do antibodies need to do?
Clear pathogens
What things need to be considered when antibodies clear pathogens?
Different pathogens, different sites of the body and replicating pathogens
What are different types of antibodies called?
Isotypes
What happens to the B cell response?
It changes during the course of the immune response
What is the BCR?
Membrane bound and IgM
What does Ig mean?
Immunoglobulin=antibody
What are other isotopes important for?
Different times of the immune response, different sites of infection and different types of pathogen
What is the first isotope?
IgM
What is the structure of IgM?
Pentamer (5 antibodies linked together by a J chain)
What is the distribution of IgM?
First Ig class produced after initial exposure to antigen. Expressed on naive B cells
What is the functions of IgM?
Very effective in activating complement, targets extracellular bacteria and acts as an antigen receptor (BCR)
What is IgG the main isotope for?
Pathogens
What is the structure of IgG?
Monomer (singular antibody)
What is the distribution of IgG?
Most abundant Ig class in blood
What is the function of IgG?
Opsonises/neutralises, only Ig class that crosses placenta = provides passive immunity and targets virus/bacteria
What is IgA the isotope for?
At mucosal sites
What is the structure of IgA?
Dimer (2 antibodies connected by secretory component and J chain)
What is the distribution of IgA?
Present in secretions such as tears, saliva, mucus and breast milk. Monomeric form in the blood
What is the function of IgA?
Defence of mucous membranes (including gut), present in breast milk, confers passive immunity on nursing infant and targets virus/bacteria
What is IgE the isotope for?
Helping destroy parasites
What is the structure of IgE?
Monomer
What is the distribution of IgE?
Present in the blood at low concentrations
What is the function of IgE?
Immunity to multicellular parasites, activates mast cells that blow up parasites and allergic reactions (e.g. to pollen or eggs)
What is IgD?
A bit weird
What is the structure of IgD?
Monomer
What is the distribution of IgD?
Expressed on naive B cells
What is the function of IgD?
Can act as an antigen receptor (BCR) and its specific function is unknown
what leads to B cell differentiation into plasma cells?
Stimulation of B cells by antigen and help from T cell
What also results from stimulation of B cells?
A small number of stimulated B cells form a pool of memory cells
what do memory cells do?
Persist for years in the blood and lymph
What do memory B cells express?
Antibody as BCR but do not secrete antibody
What happens if memory B cells see antigen again?
They respond rapidly to become plasma cells
What happens in the primary immune response?
Naive B cells activated
What is the duration of the primary immune response?
Takes around 7-14 days before sufficient antibody is produced to eliminate pathogen
What is the amount of antibody produced in the primary immune response?
Relatively low amount - mainly IgM
What does the secondary immune response rely on?
Memory B cells
What is the speed of the secondary immune response?
Fast: 2-3 days, sufficient antibody is produced to eliminate the pathogen - IgG
What is the amount of antibody produced in the secondary immune response?
Higher amounts of antibody produced
What is the secondary immune response the basis of?
Vaccination
