MCD – Immunology 5 - B-lymphocytes
Anil Chopra
1. Describe the process of stimulation of individual B cells to divide and secrete
antibody such as to generate immunity to a particular antigen (clonal selection).
There are 2 types of adaptive response
 Humoral  B-cells secreting antibodies.
 Cell Mediated  T-cells secreting cytokines, lysis.
B- cells are produced and mature in the bone marrow and travel to the lymphoid
tissue in the circulation. They form part of the humoral adaptive immune response in
the production of antibodies and memory cells. They only become activated by
antigens.
Mature B cells contain receptors specific for certain antigens.
The B-cell antigen receptor is made up of 3 parts:
- mIg
- Ig
- Ig
The mIg part of the receptor contains the
binding site (which binds to the antigen)
known as the antigenic determinant or
epitope.
There are thousands of receptors on the
surface of each B cell.
2.
2. Briefly outline the principles of immunoglobulin (Ig) gene rearrangement in the
generation of diversity.
We have a very large number of different antigens we can possibly encounter and so
we have a very large number of different B-cell receptors; this is called the
repertoire.
This diversity is generated by the way in which the B-cell receptor is synthesised.
Instead of there being one main functional gene that codes for the production of the
B-cell receptor, there are many different genetic fragments darted about the genome
on different chromosomes. When the B-cell is maturing, these gene segments are
brought together in a random order and so a different B-cell receptor is synthesised in
each cell. This is known as immunoglobulin gene rearrangement.
3. Outline the differences in antibody production during primary and secondary
immune responses.
When antigens are encountered, there are 2 main events that take place. The first is
clonal selection. This is where the B-cell receptor first encounters the antigen; the
antigen has “selected” that particular B-cell because of its specific receptor.
However, in order for activation to occur their needs to be an accessory signal, either
from
 Microbial constituents (results in production of IgM only and NO
MEMORY). – Thymus independent activation.
 T-helper cell (results in production of all Ig classes and MEMORY) –
Thymus dependent activation.
In thymus independent activation, the B-cells “express” the antigens on their surface
so that T-helper cells can bind to them and bring about the second response:
Clonal expansion – this is where at particular B-cell makes many copies of itself –
clones. These differentiate into memory cells and plasma cells. The plasma cells are
the sites of:
Antibody production – production of soluble B-cell receptors in response to the
antigen.
When the T-helper cells are activated, they release cytokines. Each of the different
cytokines stimulates/inhibits the production of the different Ig isotypes.
The memory cells that are produced have a long life span and form the basis of
immunological memory. This means that if the person encounters the same antigen
again, the secondary immune response will occur. In this response, antigen
production is a lot faster and more IgG is produced.
4. Differentiate between monoclonal and polyclonal antibodies.
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Polyclonal Antibodies: these are antibodies that come from different B-cell lines.
When an antigen enters the body, it has many different binding sites, where
different B-cells can attach. When they do so, they produce different types of
antibodies. This is a polyclonal response.
Monoclonal antibodies: are ones that are derived from one B-cell clone.
Myeloma = cancerous plasma cells that divides permanently
without antigenic stimulation and secretes antibodies which are
indistinguishable from normal antibody = myeloma proteins