Which cell provides the necessary 'OK' signal for B cell activation after antigen binding?

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B cell activation in humoral immunity usually needs two signals: the B cell’s receptor recognizing the antigen, followed by a helper T cell delivering a second, essential cue. After the B cell binds antigen and presents peptide on MHC II, a CD4+ helper T cell recognizes it and provides the key boost through two main actions: the CD40 ligand on the T cell binds CD40 on the B cell, and the T cell secretes cytokines such as IL-4, IL-5, and others. This interaction tells the B cell to proliferate, undergo class switch recombination, and form germinal centers for affinity maturation, ultimately producing high-affinity antibodies and memory B cells.

Dendritic cells are great at activating T cells in the first place, but the signal that actually “okays” B cells after antigen binding comes from the helper T cell, not from dendritic cells. Cytotoxic T cells and natural killer cells don’t provide this B cell–specific co-stimulatory help.