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A B cell has internalized a toxin that contains the epitope recognized by its surface Ig. Describe how this cell would be activated, the cellular and molecular interaction(s), and the various steps and factors leading to an appropriate immune response to clear the toxin

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Immunology has always been one of my favourite subjects, how nice for you to be taking it as a graduate course. The complexity of the B-cell response to antigens is just one of the many amazing mammalian adaptations to potentially fatal invasions of the body by foreign agents. I will outline a B-cell response step by step, starting with engulfment of the toxin by receptor-mediated endocytosis. The engulfed toxin is itself soluble and is considered an "exogenous" antigen because it was produced "outside" the body. Endogenous antigens are generated inside the body, and are comprised of epitopes from things like viruses or bacteria. This distinction is important because the immune response differs in each case.

First, let's review the details of B-cells:

Each individual has thousands of different B-cells. Each B-cell in turn recognizes a distinct, specific protein antigen epitope, a feat that is achieved through the diversity of B-cell receptors (surface Immunoglobulin or Ig). The B-cell receptor is an integral membrane protein (ie. it has a heavy chain that has many hydrophobic amino acid residues and is therefore able to embed in the plasma membrane). Thousands of copies of the B-cell receptor are present on the surface of the B-cell. They are generated before the cell ever encounters an antigen and are encoded by genes assembled by the recombination of segments of DNA in the nucleus of the ...

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This solution details the types of infections that are common in three types of inherited immunodeficienies: complement C8 deficiency; X-linked agammaglobulinaemia (Bruton's disease); and selective IgA deficiency. It explains the underlying basis of why these patients are susceptible to these particular infections.

For each of the following conditions:
1) complement C8 deficiency: a defect that results in the functional absence of the eighth complement cascade. C8 is involved in the complement lytic pathway.
2) X- linked agammaglobulinaemia (Bruton's disease) :This affects an enzyme that is required for B cell development so pre B cells fail to mature. The effect is very low levels of all classes of antibody, although T cells mature normally. The condition is X- linked and therefore predominantly affects boys.
3) Selective IgA deficiency: This is one of the more common immunodeficiencies , affecting around 1 in 700 individuals. Those affected have low IgA levels, but other immunoglobulin classes are normal as are T cells and phagocytes.

A) For each condition state what types of infection you expect each of the above conditions the affected individual would be susceptible to.
B) For each condition suggest why an individual would be susceptible to the types of infections you listed in the question above (A).

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