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CD40, a member of the TNF receptor superfamily, is a glycoprotein crucial for immune responses. It's key for T cell-dependent processes like immunoglobulin class switching, memory B cell development, and germinal center formation. Expressed on B cells, dendritic cells, macrophages, and some tumor cells, CD40 interacts with CD40L to initiate signaling pathways. This interaction is vital for humoral immunity. CD40 defects can lead to hyper-IgM immunodeficiency type 3 （HIGM3）, causing issues in B cell isotype switching and antibody responses. CD40's extracellular domain has cysteine-rich repeats, typical of TNF receptors. Early studies emphasized its role in humoral immunity.