B cells and Antibody Flashcards
what do B cells recognise
whole (naive) protein antigens
the antigens they respond to don’t need to be processed into peptides
what are B cells activated by
recognition of antigen
CD4+ help - cytokines
most B cells in the body aren’t producing antigen, only the ones that are activated and have been called on.
what is a B cell?
are lymphocytes that develop in the bone marrow and become mature in the bone marrow
express unique antigen receptors (BCR or secreted antibody)
plasma cells are activated by B cells that secrete antibody memory B cells provide “memory” - they float around waiting for antigen to come around a second time
they circulate from lymph node to thymus to bone marrow and keep circulating looking for pathogens
BCR
almost all B cells express a unique B cell receptor
this enables the recognition of a wide variety of antigens - this happens when the DNA gets rearranged and this causes a wide range
germline state
when the receptor gene is not activated so the B cell doesn’t have a receptor on it.
Shape of antibody/ BCR receptor
has 2 identical light chains and 2 identical heavy chains
the antigen binding sites are are made of he Variable regions of the light and heavy chain on both sides. heavy chains come together at the end to make the Fegion which is the end that sticks into the B cell but sticks out of the antigen as a tag
BCR
B cell receptor
the surface of each B cell is covered with around 100,000 BCR. mainly IgM/ IgD
the BCR binds to antigen and activated the B cell
3 functions of antibody
- neutralisation
- opsonisation
- complement activation
Neutralisation
antibody binds to surface structures on microbes blocking their ability to attach to host cells
antibody binds to toxins preventing them from acting on host cells
Opsonisation
antibody binds to surface of pathogen
antibody facilitated uptake by phagocytosis
antibody have 2 binding sites and can agglutinate (firmly stick) bacteria. this increases he efficiency of uptake
Complement activation
antibody binds to surface of pathogens
bound antibody activated the complement pathway
ultimately C proteins form a membrane attack complex (MAC) destroying the microbe.
IgG
monomer most abundant Ig class in the blood opsonises/ neutralises only Ig class that crosses placenta provides passive immunity targets virus/ bacteria
IgA
dimer
present in secretions such as tears, saliva, mucus and breast milk
monomeric form in the blood but when secreted it is in the dimer form.
defence of mucous membranes
present in breast milk
confers passive immunity on nursing infant
targets virus/ bacteria
IgM
pentamer when secreted first Ig class produces after initial exposure to antigen expressed on naive B cells very effective in activating complement targets extracellular bacteria acts as antigen receptor (BCR)
IgE
monomer when secreted present in blood at low concentration immunity to multicellular parasites allergic reactions also activates mast cells for parasite immunity and the allergic response
