Immunology 2: Antibodies Flashcards Preview

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Flashcards in Immunology 2: Antibodies Deck (17):

What is the structure of an antibody?

  • Each antibody is made up of (at least) four antibody chains: two identical heavy chains, and two identical light chains

  • Each chain has a constant region and a variable region. Constant regions show little variability, but there are millions of different variable regions. 

  • The variable region of an antibody determines specificity

  • Antibodies are secreted by B cells and are also found on their surface (around 200k, all identical)


Observe the structure of IgG1. Do you understand it?

  • The variable region of an antibody determines specificity
  • The Fragment crystallisable (Fc) region determines the antibody class and function


What the five major antibody classes?

Antibodies are classified by the five major variants of the Fc region.

  • IgG (4 subclasses), IgM, IgA (2 subclasses), IgE, IgD


While the variable regions of antibody determines its specificity, the Fc region determines the anitbody class and function. What are some examples of Fc mediated function?

  • Complement fixation (triggers complemenent cascade) involves interaction between Fc region of aggregated antibodies and complement proteins
  • Many cells express Fc receptors


Elaborate on Phagocytosis induced by Fc reception:

  • No effective binding of antibodies and Fc receptors on macrophages until clustering occurs.
  • When antibodies are not clustered, they will only interact weakly with Fc receptors
  • When antibodies are clustered (i.e. bound to a bacterium), there are multiple interactions between Fc regions and Fc receptors, this leads to stable binding - activation of macrophage leading to phagocytosis and destruction of bacterium


What is the structure and function of IgM?

  • secreted IgM has a pentameric structure (five identical chains united by the J chain – 10 antigen binding sites)
  • Monomeric IgM is found on B cell surface
  • Activates complement well
  • Good agglutinator of antigen (binds multiple bacteria at the same time due to five arms)
  • Prominent in the primary response (can bind stably to bacteria which it fits imperfectly due to different arms binding multiple sites on the bacteria)


What is the structure and function of IgG (1)?

  • 4 sub classes (see structure of IgG1 above)
  • Abundant in serum and body fluids
  • Longest half-life in serum – approx. 30 days
  • Prominent in the secondary response
  • lgG1 is good activator of complement
  • lgG1 binds to Fcγ receptors on many cells
  • lgG1 crosses the placenta to provide protection to the newborn – slowly fades after birth as the babies own immune system ramps up, lowest AB presence at 3 months
  • Although there is no explanation for the different subclasses, they act in this order:
    • 3, 1, 2, then finally if antigen  persist 4


What is the structure and function of IgA?

  • Abundant in secretions (milk, saliva, tears and secretions of the GIT and genitourinary tract) IgA is very important in the mucosal defences
  • Exported by plasma cells as dimers (with a J chain)
  • A fragment of the plgR (secretory component) remains bound to secreted lgA, conferring resistance to proteolysis
    • gives some stability to the molecule (important as IgA in the GIT is vulnerable to attack by proteases – both our own and those produced by bactera)
  • Bacteria in the gut may destroy lgA through action of proteases
  • Humans have 2 lgA sub-classes (perhaps ensures that there are antibodies that can resist bacterial protease attack)
  • Monomeric lgA is present in serum at relatively high concentrations


What is the function of IgA?

An antibody of uncertain function, which is found on the surface of naïve B cells - that is, mature B cells that have not encountered antigen


What is the function of IgE?

Is produced in very small quantities, but is very powerful. It is responsible for allergic reactions, and is prominent in the response to parasitic infections


What are the simple concepts of immunocogenics?


There are millions of different B cells with distinct specificities. Specificity is determined by the amino acid chains that constitute the antigens on the B cell (both light and heavy). Every polypeptide chain is encoded by a different gene.

There are clusters of different genes (V, D, J)

At the birth of a B cell, there is a random process of gene arrangement which brings together a V, a D and a J. These yields many combinations (45 x 23 x 6)


What is the simple point of the diversification of antibody function?

  • Downstream from the gene segments that determine specificty, is a string of constant region genes. These determine the iostype of the transcribed antibody
  • VDJ genes that encode a particualr specificity are expressed in association with a constant region genes, to give a complete antibody. Different constant region genes give the antibodies different functions.


What is class switching?

Naïve B cells (haven’t been exposed to antigens. Express both IgM and IgD on their surface. After clonal selection, with T cell help, they can switch to the production of IgA, IgG or IgE. New antibodies have the same specify (same variable region genes) but different class and function (different constant region genes)


What are, and what is the importance of constant region genes?

Genes which determine antibody isotype (constant region genes) are located downstream from those which determine specificity. Therefore antibodies produced with certain specificities can have different constant region genes (isotypes), leading to altered antibody function


What are vaccinations?

  • Vaccination is a safe way of inducing immunological memory so that on first encounter with the real infectious agent, a strong secondary-like response can be mounted
  • Vaccines teach us to respond to dangerous toxins using chemically modified (non-toxic) toxoids.

For tetanus- Exposure to a toxoid produces plasma cells that make antibodies for years. (Therefore, boosters). TETANUS WILL KILL YOU BEFORE YOUR MEMORY CELL CAN WORK.


Why does antibody production help us?

  • Immune complex formation triggers the complement cascade
  • Antibody binding to antigens 'flags' antigen, allowing interaction between the antigen and cells of the immune system
  • Steric hindrance. If antibody binds to certain parts of an antigen, this can be directly beneficial.


How can we measure antibodies?

Specific antibodies are produced to fight infection. Detection of specific antibodies is a useful way to diagnose and confirm infection (HIV). IgG (but not IgM) persists long after infection, so IgM can be used to test for recent infection. Sometimes, measurement of antibodies taken over time is useful.