Lecture 45 Flashcards
Whats the major difference between what is recognised by B cells and T cells? What is similar in the activation of CD8+ and B cells? Hence what do B cells require for activation?
B cells recognise whole (native) protein antigens, T cells recognise peptide / MHC complexes produced by breaking up those native protein antigens.
Both CD8+ T cells and B cells require cytokines released by CD4+ T cells to activate. As such B cells require both recognition of a native antigen protein and CD4+ cytokines.
What are B cells? What two forms are produced by activating a naive B cell?
An immune cell of the lymphoid (lymphocyte) lineage which are both formed and matured in the bone marrow. They express unique antigen receptors (either are B cell receptors on the cell membrane or in the secreted form as antibodies). Activation of naive B cells leads to production of plasma cells (the activated form, secretes antibodies) and memory B cells.
What are the circulation paths of the T and B cells?
stem cells become both pre-B and pre-T cells in the bone marrow. The pre-B cells become mature B cells in the bone marrow before circulating to the lymph node. The pre-T cells circulate to the thymus where they mature into T cells or get destroyed before being circulated to the lymph node if they pass.
How are B cells able to recognise a wide variety of antigens?
Individually B cells can only act against one antigen, like T cells however, each B cell possesses a statistically unique B cell receptor produced by rearrangement of the BCR genes.
Describe the structure of a B cell receptor, which part gives the function of the antibody? Which gives the antigen specificity?
In the B cell receptor state the antibody has a part which makes up the transmembrane region (the BCR is found slightly inside the cell and largely outside so that it can bind with it’s antigen). The BCR forms a Y shape, it has two identical heavy chains which are the parts which are part inside and part outside the membrane, these are joined before the seperation point be disulfide bridges. On the outer side, once the seperation point occurs there are two identical light chains which are attached by disulfide bridges to the heavy chains (one on each chain). There are two regions, known as the variable region (found at the outer end of both chains) which code for the antigen specificity and are unique to the individual B cell. Prior to these variable regions are the constant regions, which instead encode for the functionality of the antigen (there are five options for this, IgG, IgM, IgA, IgD and IgE). Note that the constant region can be changed by the B cell but the variable region cannot be changed.
What are the three functions of antibodies? What do they all have in common?
Neutralisation, opsonisation and complement activation. All of these require the antibody binding to the antigen
How do antibodies neutralise an antigen?
The antibody binds to the surface structures or toxins of the antigen, surrounding them and blocking their ability to interact with host cells.
How do antibodies perform opsonisation?
The antibodies bind to the surface of the pathogen and facilitate uptake by phagocytes. The antibody can bind to two bacteria using its two seperate binding sites to agglutinate (glue) the bacteria together, increasing efficency of uptake by phagocytes.
How do antibodies trigger complement activation?
The antibody binds to the surface of the pathogens and activates the complement pathway, ultimately leading to complement proteins forming membrane attack complexes on the microbe (killing the cell by forming pores).
What two antibody classes/isotypes are most common on naive B cells as B cell receptors and what are their functions and structures?
IgM: The first Ig class produced after initial exposure to pathogens, has a pentamer structure as an antibody but has a monomeric structure as a B cell receptor. (5 antibodies joined via links and a J chain on the non binding side.) Very effective in activating complement system, targets extracellular bacteria. IgD: expressed primarily on naive B cells but also found slightly in blood serum. Functions to act as an antigen receptor with IgM but specific functions are unknown.
What two antibody classes/isotypes provide ‘passive immunity’? What do they do and what is passive immunity?
IgG: The most abundant class of antibody in the blood, has a monomeric form and functions primarily to opsonise and neutralise antigens (primarily viruses/bacteria), is also the only Ig class able to cross the placenta, which is what allows it to provide passive immunity. IgA: Has a dimer structure (two monomers joined back to front by a J chain and has a secretory component), in the blood it has a monomeric form. Present in secretions such as tears, saliva, mucus and also breast milk (which is how it provides passive immunity), mainly provides defence of mucous membranes (especially the gut) and targets viruses and bacteria. Passive immunity is the short term immunity gained from the introduction of antibodies from another person or animal. In this case it applies to mothers giving passive immunity to their young children.
What class/isotype of antibody causes allergic reactions? What is its actual purpose meant to be? What is a common medicine which can cause this to harm the body and why?
IgE, a monomeric antibody class which is present in the blood at low concentrations, its primary function is to provide immunity to multicellular parasites but also causes allergic reactions. A prime example of this is the antibiotic penicillin, this is because penicillin has a very reactive beta-lactam ring which can modify self produced proteins, IgE can then react to these self-proteins, even breaking them up to form antigen peptides which can cause the T cells to react as well, possibly leading to side effects like hives or anaphylactic shock.
What are monoclonal antibodies? What could they be used for?
Antibodies derived from a single clone of B cells. These could be used in medicine or diagnostics for many reasons (e.g pregnancy tests) and are being researched currently.
