Overview of the B-cell functions and their potential relevance for GVHD. B cells contribute to immune responses by antibody-mediated and antibody-independent mechanisms. Antibodies produced by B cells can lead to complement activation, antibody-dependent cell-mediated cytotoxicity, and Fc-receptor antigen uptake and phagocytosis. B cells can furthermore secrete a large number of proinflammatory cytokines including IL-2, TNF-α, IL-6, IL-12, MIF, and interferon-γ that activate a large number of immune cells such as T cells (including Th17 cells), macrophages, and natural killer (NK) cells and have been shown to be involved in regulation of the GVH reaction. Antigen presentation by activated B cells that have up-regulated major histocompatibility complex and costimulatory molecules such as CD80 and CD86 leads to CD4⁺ and CD8⁺ T-cell activation and differentiation. Depending on the B-cell subset and the nature of the activation stimulus B lymphocytes can also act as immunoregulatory cells that induce peripheral CD4⁺ and CD8⁺ T-cell tolerance, inhibit dendritic cells, and induce and expand regulatory T cells.