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4 - Engineering antibodies page 2
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2. Understanding specific immunity Link to the Medical Research Council web site

Figure 2. Basic structure of an antibody (IgG) shown here binding to an antigen through the antigen binding regions. The constant regions (white) trigger complement and cell mediated killing. The amino acidssequence of the constant regions is the same for each class of antibody (IgA, IgD, IgE, IgG and IgM). The variable region has an amino acid sequence that is unique for a given antibody.
What is an antigen?
Antigens are proteins, glycoproteins or polysaccharides. They may be free molecules, or they may be present on the surface of an invading microbe, or on the cell membranes of a transplanted organ, such as a replacement kidney. Antigenic molecules are present on all cells, but they usually provoke an immune response only if they are foreign.
Taking a look at antibodies
Antibodies are glycoproteins. They all have a similar basic structure (see Figure 2) and are known collectively as immunoglobulins (often written as 'Ig' for short).

Antibodies are synthesised by a type of white blood cell called a B cell and are displayed on the surface of the cell. Each B cell is genetically programmed to display only one specific antibody.

B cells arise through the differentiation of stem cells in the bone marrow. During this process, extensive rearrangements of the antibody genes occur to produce several million different variants. Each unique genetic sequence then codes for the production of a specific antibody. The number of different antibodies is so large that at least one will recognise a foreign antigen, even if the body has never previously been exposed to it

The B cell may be described as a 'genetic display package', with the antibody on the outside where it can meet and bind with the antigen, and with the genes which code for the antibody within.

Figure 3

Figure 3. Activation of a B cell by a foreign antigen from a microbe.

 

 

 

What happens when a B cell binds its antigen?
Look at figure 3. Antigens from microbes trigger an immune response. They activate specific B cells to divide many times. The B cells proliferate and differentiate into either short-lived plasma cells, which make and release soluble antibodies, or long-lived memory cells with surface antibody. Each antibody molecule shows a specific affinity ('liking for') its particular antigen. The higher the affinity the more readily the antigen is bound and the more efficient the immune response.

Repeated contact with an antigen acts as a selection pressure for the mutation of the antibody genes. This results in the generation of high affinity antibodies.

 'I remember you'
A characteristic feature of specific immunity is the generation of immunological memory through the production of memory cells (see Figure 3). Exposure to an antigen leads to the production of antibodies: the primary response. On subsequent encounters with the same antigen, a secondary response is observed. Immunological memory forms the basis of all vaccination procedures.

Question 2

Look at the graph in Figure 4. Use it to describe and explain the features of a typical primary and secondary response to an antigen.



Figure 4.


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