CD40 is a surface antigen expressed on B cells. The CD40 ligand (CD40L) is expressed on activated T cells. Interaction between CD40 and CD40L is critical for proliferation and isotype switching in the context of a response to a T-cell-dependent antigen. Patients with X-linked hyper-IgM syndrome (HIGMX-1) in their CD40L gene are unable to switch from IgM to IgG, IgA and IgE. Mice with a disrupted CD40 gene fail to undergo isotype switching to T-cell-dependent antigens but respond normally to T-independent antigens.
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