What is Class Switching or Isotype Production after activation of the B Cell?

Initially, the constant chain that is expressed will be a class mu chain. Since p is the first constant region of gene to be encountered, B lymphocytes make IgM class antibodies at first, then class switching takes place to go well with the requirement and antigen encountered.

Class switching or Isotype production is a biological process occurring after activation of the B cell. This mechanism allows the cell to produce different classes of antibodies (IgA, IgE, or IgG).

The different classes of antibodies, and their effector functions, are defined by the constant (C) regions of the immunoglobulin heavy chain. Each isotype is adapted for a distinct function, therefore, after activation, an antibody with IgG, IgA, or IgE effector function might be required to effectively eliminate an antigen.

Class switching allows different daughter cells from the same activated B cell to produce antibodies of different isotypes. Only the constant region of the antibody heavy chain changes during class switching. The variable regions, and therefore antigen specificity, remain unchanged.

Thus the progeny of a single B cell can produce antibodies, all specific for the same antigen, but with the ability to produce the effector function appropriate for each antigenic challenge.

Class switching is triggered by cytokines; hence the isotype generated depends on the cytokines present in the B cell environment.

Class switching occurs in the heavy chain gene locus by a mechanism called class switch recombination (CSR). This mechanism relies on conserved nucleotide motifs, called switch (S) regions, found in DNA upstream of each constant region gene (except in the 5-chain).

The DNA strand as shown in the figure is broken by the activity of a series of enzymes at two selected S-regions. The variable domain exon is rejoined through a process called non-homologous end joining NHEJ) to the desired constant region (7, a or e). This process results in an immunoglobulin gene that encodes an antibody of a different isotype.

Antibody Production Rate and Quality:

The rate of antibodies production and their quality depends upon the antigens responsible for their production.

High concentration of new antigens initiates early differentiation and proliferation of B cells. When the concentration of antigens is low, the B cells differentiation is also fairly low but the antibodies produced by the plasma cells show high affinity towards their antigens.

The time availability in the differentiation process might allow the proper rearrangement of genes responsible for the production of variable domains to generate high affinity (highly specific) binding regions in antibodies.