B-cell activation and differentiation

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B-cell activation and differentiation


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USMLE® Step 1 questions

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High Yield Notes

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B-cell activation and differentiation

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USMLE® Step 1 style questions USMLE

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A researcher isolates an immune cell from a blood sample. This cell initially produces IgM but, after being stimulated by a T-cell, can go on to produce other classes of antibodies. Which of the following surface proteins is most likely expressed by this cell?  

External References

First Aid








“Fried egg” plasma cell p. 438

Golgi apparatus p. 45

in plasma cells p. 412

Multiple myeloma p. 438

as plasma cell cancer p. 412

Plasma cells p. 414

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The immune response is highly specific for each invader, and that’s because the cells of the adaptive immune response have unique receptors that can differentiate friendly bacteria from potentially deadly pathogens from their unique parts - called antigens.

The key cells of the adaptive immune response are the lymphocytes - the B and T cells.

B cells develop in the bone marrow where they undergo a process called VDJ rearrangement to generate a massively diverse set of B cell receptors.

The B cell receptor is essentially an antibody except that it has a transmembrane part that goes through the membrane attaching the receptor to the surface of the B cell.

The B cell receptor, has two heavy chains and two light chains, and the region or fragment of the B cell receptor that binds the antigen is called the fragment-antigen binding or Fab region.

The Fab region is where the ends of the heavy and light chains meet, and there are two Fab fragments on each B cell receptor.

The remainder of the heavy chain makes up the constant region or constant fragment region, also called Fc.

The two heavy chains are linked to one another by disulfide bonds and each heavy chain is also linked to a light chain by a disulfide bond.

Each B cell receptor, has two identical heavy and light chains, resulting in two identical antigen binding sites.

As the B cell develops into a plasma cell, the B cell receptor gets secreted as an antibody with the exact same antigen specificity.

However, the heavy chain actually changes as the B cell develops.

There are 5 major types of heavy chains which encode the isotypes or classes of immunoglobulins: IgM, IgD, IgG, IgA, and IgE.


B cells are activated when they encounter an antigen that they recognize. The antigen binds to the B cell's surface receptors, which activates and triggers it to divide and differentiate into an antibody-secreting plasma cell. Plasma cells produce antibodies that bind to the antigen and neutralize it.

The differentiation process is controlled by various factors, including cytokines, lymphokines, and chemokines. Each of these molecules signals the B cells to differentiate into a certain type of plasma cell.


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