TNF and infection: The MHC also harbors genes involved in antigen processing and loading (HLA-DOA, HLA-DOB, HLA-DMA, HLA-DMB, TAP1, and TAP2), cytokines such as tumor necrosis factor (TNF) and complement components, and genes that modulate the activity of NK cells, as MICA and MICB. The expression of MICA and MICB is low in normal tissues but is induced in tumors or during infections, upregulating (when expressed in the membrane) or downregulating (when expressed as soluble isoforms) NK cell cytotoxic activity (13).