Lecture 35 - B cells and antibody Flashcards
Dendritic cells purpose
Dendritic cells are the link between the innate and adaptive systems. Dendritic cells phagocytose a pathogen and then carry information about it to our adaptive immune system cells. This information is produced and shared in the form of antigens (antigens are the traces that pathogens leave behind, they are molecules found on the surface of pathogens that can be detected by our adaptive immune system for recognition). Dendritic cells pass this information on to our T cells
MHC
Major histocompatibility complex - launches two different arms of the immune system - the cytotoxic arm and the antibody mediated arm
B cells
B cells are lymphocytes that develop in the bone marrow. B cells mature directly in the bone marrow and do not have to go to the thymus like the T cells do.
Express unique antigen receptors (BCR or secreted antibody) undergo genetic rearrangement similar to the T cells which creates a huge diversity and usually no two B cells are alike)
Plasma cells are activated B cells that secrete antibody (B cells make antibody which it becomes a plasma cell and only when it gets signalled to help or recognition of a native antigen)
Memory B cells provide ‘memory’
Origin of B and T cells
Both B and T lymphocyte precursors originate in the red bone marrow
Maturation of B and T cells
Lymphocyte precursors that are destined to become T cells migrate (in the blood) to the thymus and mature there
B cells mature in the bone marrow
During maturation lymphocytes develop immunocompetence and self tolerance
B cell receptor structure
Two identical heavy chains and two identical light chains. Both contribute to antigen binding. The 4 chains combine to form a molecule called an antibody monomer with two identical halves.
BCR is embedded in the membrane of the B cell therefore capable to signalling into the B cell. Important for the B cell so that it can actually sense if there is actually something bad out there, also needs help from CD4 helper T cells to become activated.
Two antigen binding sites on BCR which will attach 2 antigens of the same pathogen
Transmembrane region of the BCR
Antigen binding site - The variable regions of the heavy and light chains combine to form an antigen-binding site which is shaped to fir a specific antigenic determinant
B cell receptor
The surface of each B cell is covered with approximately 100,000 BCR (mainly IgM and IgD antibodies)
The BCR binds antigen and activates the B cell
Naive but not activated (not making antibody) B cells have BCRs plastered around its plasma membrane sticking out and they are a mixture of IgM and IgD variety. The type of antibody is determined by the constant region expressed is either IgM or IgD, constant regions are not involved in binding the antigen but impart different functions on antibodies.
Plasma cells are
Differentiated B cells
Light chain
Are identical and are about half as long as the heavy chains
Heavy chains
Two chains that are identical to each other
Antigen binding site
The variable regions of the heavy and light chains combine to form an antigen-binding site which is shaped to fir a specific antigenic determinant
Naive B cells
A naive B cell is a B cell that has not been exposed to an antigen. Once exposed to an antigen, the naive B cell either becomes a memory B cell or a plasma cell that secretes antibodies specific to the antigen that was originally bound.
Three functions of antibody
Neutralisation
Opsonisation
Complement activation
Viral neutralisation
Virus usually has to attach to a cell receptor to infect and a neutralising antibody will block this from happening rendering it inactive
Blocking the pathogen or toxigenic products from affecting our host cells
Opsonisation
Making tasty
Bacteria covered in antibody that makes it more tasty for a phagocyte such as macrophages.
For example - the macrophage has receptors for antibody and they are particular active when the antibody is bound to the antigen so the macrophage is able to take this bacteria up because it has receptors for the antibody when it is bound
Activation of complement system and pore formation
Classical way that the innate system measures together with the adaptive system
Antibody bound to the foreign cell will very quickly activate complement at the cell surface which not only result in opsonisation by the antibody and complement proteins but also the formation of MAC (membrane attack complex) which makes in the foreign cell which causes them to burst due to the flow of water and ions into the cell
IgG
Most abundant IgG class in the blood (or antibody in the blood)
Monomer
Function
Opsonises/neutralises
Only Ig class that crosses placenta - provides ‘passive immunity’ (to the unborn child in utero)
Targets virus/bacteria
If you have had a vaccination that has been successful you would expect to find a lot of IgG
IgA
Dimer (monomer exists in limited amounts in the plasma)
Has a secretory component which can be secreted to mucosal surfaces and has a J-chain
Distribution
Present in dimeric form in secretions such as tears, saliva, mucus, and breast milk
Monomeric form in the blood
Function
Defence of mucous membranes especially gut
Present in breast milk
Confers ‘passive immunity’ on nursing infant
Targets virus/bacteria
J chain
The joining (J) chain is a small polypeptide, expressed by mucosal and glandular plasma cells, which regulates polymer formation of immunoglobulin (Ig)A and IgM.
