During an immune response, B lymphocytes can switch expression of immunoglobulin (Ig) class (isotype) from IgM to IgG, IgE, or IgA. This Ig class switch is based on a deoxyribonucleic acid (DNA) recombination event that results in an exchange of the gene segments coding for the constant region of the Ig heavy chain, although the Ig heavy chain variable region is retained. This process changes the effector functions of the corresponding antibody (Ab). Much of our current understanding of the molecular mechanisms of class switch recombination is based on the analysis of B cells induced to switch class of Ig in vitro. In vitro, murine and human naive B cells can be activated with bacterial lipopolysaccharides, anti-CD40 or CD40L, to undergo class switch recombination. Cytokine signals can direct class switch recombination to distinct classes; for example, interleukin-4 will target murine IgG1 and IgE, and human IgG4 and IgE. Here we describe the technologies for the isolation of B lymphocytes, their activation to class switching, and the analysis of Ig class switching.