B cells and Antibody Flashcards
What cells produce antibody and how?
B cells called plasma cells are activated B cells that secrete antibody
B cells:
- They are lymphocytes that develop in bone marrow
- Express unique antigen receptors (BCR or secreted antibody)
- Plasma cells are activated B cells that secrete antibody
- Memory B cells provide ‘memory’
Describe the B cell receptor
- The surface of each B cell is covered with ~100,000 BRC (mainly IgM/IgD antibodies)
- The BCR binds antigen and activates the B cell
- BCR is membrane anchored via a transmembrane domain (TM). Secreted antibodies lack TM
What are the three functions of antibody?
- Neutralisation:
- Effective against toxins
- Stops viruses and bacteria from latching on (viral neutralisation) - Opsonisation:
- Labels bacteria for uptake by phagocytosis by coating them in antibody.
- Phagocytes often have receptors for antibody - Compliment activation
- IgM and IgD are good at activating compliment
- When antibody binds to cell surface, it can result in formation of a MAC
Describe antibody binding to defined regions (antibody binding sites) on a larger structure
- Antibodies bind native antigens. Several different antibodies may target a single type of microbe
- The term native antigen means that the antigen does not have to be processed to peptide (or in context of MHC)
- Antibodies can recognise just about any structure (very flexible in what structures they can recognise)
Antibody binding site = epitope
Describe immuno-globulin (Antibody) class G (IgG) (monomer)
Distribution:
- Most abundant Ig class in blood
Function:
- Opsonises/neutralises
- Only Ig class that crosses placenta: provides ‘passive immunity’
- Targets virus/bacteria
Passive immunity is getting antibodies from an external source
Describe immuno-globulin (Antibody) class A (IgA) (dimer)
Distribution:
- Present in secretions such as tears, saliva, mucus, and breast milk
- Monomeric form in blood
Function:
- Defence of mucous membranes, especially gut
- Present in break milk
- Confers ‘passive immunity’ on nursing infant
- Targets virus/bacteria
Describe immuno-globulin (Antibody) class M (IgM) (pentamer)
Distribution:
- First Ig class produced after initial exposure to antigen
- Expressed on naive B cells
Function:
- Very effective in activating Compliment
- Targets extracellular bacteria
- Acts as antigen receptor (BCR)
Describe immuno-globulin (Antibody) class E (IgE) (monomer)
Distribution:
- Present in blood at low concentrations
Function:
- Immunity to multicellular parasites
- Allergic reactions
IgE activates mast cells for parasite immunity and the allergic response
Describe immuno-globulin (Antibody) class D (IgD) (monomer)
Distribution:
- Expressed on naive B cells
Function:
- Together with IgM, acts as antigen receptor (BCR)
- Specific function unknown
Describe memory responses and memory cells
Memory responses:
- Stimulation of B cells by antigen + T cell leads to formation of plasma cells
- In addition, a small number of stimulated B cells form a pool of memory cells
Memory cells:
- Memory cells persist for years in blood and lymphatic tissue
- Express antibody as BCR, but do not secrete antibody
- Respond rapidly to antigen encounter and become plasma cells
Describe primary immune responses
- Takes around 7-14 days before sufficient antibody is produced to eliminate pathogen
- Relatively low amount of antibody produced - mainly IgM
Describe secondary immune responses
- Basis of the success of vaccination
- Relies on memory B cells
- Fast: 2-3 days, sufficient antibody is produced to eliminate pathogen - mainly IgG, with additional class switching to IgA and IgE (low levels)
How do immuno-globulin molecules (Antibody) get their function/class?
The constant region of the Ig molecule switches so they have a new function (become a new class). These gene rearrangements to make different types of B cells happen in the bone marrow.
