Class switch

Class switching Change in the isotype of antibody produced after a B cell has encountered an antigen. 

FUNCTION MEDIATES B-CELL PROLIFERATION IN THE ABSENCE OF COSTIMULUS AS WELL AS IGE PRODUCTION IN THE PRESENCE OF IL-4. INVOLVED IN IMMUNOGLOBULIN CLASS SWITCHING. 

Petersen S, Casellas R, Reina-San-Martin B, Chen HT, Difilippantonio MJ, Wilson PC, Hanitsch L, Celeste A, Muramatsu M, Pilch DR, Redon C, Ried T, Bonner WM, Honjo T, Nussenzweig MC, Nussenzweig A (2001) AID is required to initiate Nbsl/gamma-H2AX focus formation and mutations at sites of class switching. Nature 414 660-665... 

As compared to DCs, B cells are very poor APCs and play a major role as source for antibodies. Upon stimulation by antigens and in the presence of T cells at the border of the T-cell-B-cell area, adjacent to follicles, B cells become antibody-secreting cells and eventually form a germinal center (GC) response. GCs are specialized follicles for B-cell expansion, somatic hypermutation, and class switch recombination, processes that are regulated by T cells, follicular DCs, and other cells. In this process of B-cell maturation, Tregs seem to play a critical role, as in several immune diseases, which are characterized by aberrant antibody... 

However, recent data obtained in humans also suggest direct actions of Tregs on B cells, as Lim et al. [31] have shown that isolated Tregs can suppress antibody production and the class switching recombination of B cells in the absence of T-helper cells. 

Th2 cells The Th2 cells secrete cytokines (IL-4, -6 and -10) that stimnlate B-cells. They can cause a class switch from IgG to IgE antibodies. The latter activate eosinophils, which are important in killing parasites that are too large to be phagocytosed. Unfortnnately, they are also involved in allergic reactions (see below). 

Once the specificity of the antibody has been established, it can be further characterized with respect to it isotype. This may be relevant to its application, but is also important if class switch variants are to be detected. 

Yasui, H., Akahori, Y., Hirano, M., et al. (1989). Class switch from m to d is mediated by homologous recombination between sm and Sm sequences in human immunoglobulin gene loci. Eur. J. Immunol., 19, 1399-1403. 

Takhar P, Smurthwaite L, Coker HA, Fear DJ, et al. 2005. Allergen derives class switching to IgE in the nasal mucosa in allergic rhinitis. J Immunol. 174 5024-5032. 

---