B cells that recognise a primary encounter with a pathogen are called memory B cells. B cells can survive for a very long period of time in the body and as soon as the body encounters the same pathogen again they are able to react immediately against the pathogen. This is the reason why the second encounter is usually stronger and at a faster pace. Certain vaccines also work on this property of B cells. They introduce weakened or dead pathogens so that the body is produce memory B cells and on subsequent encounters the body is able to recognise the pathogen and act fast. 

State University of New York at Buffalo

B lymphocytes, which are commonly called B cells, express surface B cell receptors for antigen immunoglobulins.  B cells are further differentiated based on which kinds of antigens they respond to and which type of antibodies they produce. There are two different types of B cells:
1. Plasma B cells
2. Memory B cells

B Lymphocytes

Plasma B cells produce antibodies (also called immunoglobulins) that bind to the antigens which makes it easier for phagocytic cells to target and kill them. There are five classes of immunoglobulins:
1. IgG
2. IgM
3.IgA
4. IgE
5. IgD

Function of Plasma B cells

Function of Memory B cells

- Antigens (foreign substance) stimulate creation of preexisting B cell the antibodies which recognize the antigen. 
- The genes that code for antibodies are limited – so where does the diversity come from? 
- The antibody diversity is accounted from the genetic changes of the B lymphocytes development. 
- Antibodies are coded by DNA segments: V, J and C for the light chains and V, D, J and C for the heavy chains. Because there are so many segments and many versions of these segments are possible, the possibilities for antibodies are significantly large. 
- Additionally, changes occur once a B cell has encourntered an antigen and secrets antibodies. 
- The B cells divide and the antibody gene mutation increases and creates a trial of nucleotides that vary (one amino acid mutations for every cell generation) and this allows for antigen specificity of the antibody to be fine tuned over many cell generations. This antigen-binding affinity increases with the B cell proliferation. 


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