MASP-1, MASP-2, MASP-3 Serine proteases of the classical and lectin pathway of complement activation that bind to C1q, ficolins, and mannosebinding lectin, and function in their activation to cleave C4.
mast cells A large granule-rich cell found in connective tissues throughout the body, most abundantly in the submucosal tissues and the dermis. The granules store bioactive molecules including the vasoactive amine histamine, which are released on mast-cell activation. Mast cells are thought to be involved in defenses against parasites and they have a crucial role in allergic reactions.
mastocytosis The overproduction of mast cells.
mature B cell B cell that expresses IgM and IgD on its surface and has gained the ability to respond to antigen.
MAVS (mitochondrial antiviral signaling protein) A CARD-containing adaptor protein attached to the outer mitochondrial membrane that signals downstream of RIG-I and MDA-5 to activate IRF3 and NFκB in response to viral infection.
MBL-associated serine proteases See MASP-1, MASP-2, MASP-3.
M cells Specialized epithelial cell type in the intestinal epithelium over Peyer’s patches, through which antigens and pathogens enter from the gut.
MD-2 Accessory protein for TLR-4 activity.
MDA-5 (melanoma differentiation-associated 5, also helicard) This protein contains an RNA helicase-like domain similar to RIG-I, and senses double-stranded RNA for detection of intracellular viral infections.
medulla The central or collecting point of an organ. The thymic medulla is the central area of each thymic lobe, rich in bone marrow-derived antigen-presenting cells and the cells of a distinctive medullary epithelium. The medulla of the lymph node is a site of macrophage and plasma cell concentration through which the lymph ﬂows on its way to the efferent lymphatics.
MEK1 A MAPK kinase in the Raf–MEK1–Erk signaling module, which is a part of a signaling pathway in lymphocytes leading to activation of the transcription factor AP-1.
melanoma-associated antigens (MAGE) Heterogeneous group of proteins of diverse or unknown functions characterized by restricted expression limited to tumors (i.e., melanoma) or testis germ cells.
membrane associated ring finger (C3HC4) 1, MARCH-1 An E3 ligase expressed in B cells, dendritic cells, and macrophages that induces the constitutive degradation of MHC class II molecules, regulating their steady-state expression.
membrane attack Effector pathway of complement based on formation of the membrane-attack complex (MAC).
membrane-attack complex (MAC) Protein complex composed of C5b to C9 that assembles a membrane-spanning hydrophilic pore on pathogen surfaces, causing cell lysis.
membrane cofactor of proteolysis (MCP or CD46) A complement regulatory protein, a host-cell membrane protein that acts in conjunction with factor I to cleave C3b to its inactive derivative iC3b and thus prevent convertase formation.
membrane immunoglobulin (mIg) Transmembrane immunoglobulin present on B cells; it is the B-cell receptor for antigen.
memory B cells See memory cells.
memory cells   B and T lymphocytes that mediate immunological memory. They are more sensitive than naive lymphocytes to antigen and respond rapidly on reexposure to the antigen that originally induced them.
mesenteric lymph nodes   Lymph nodes located in the connective tissue (mesentery) that tethers the intestine to the rear wall of the abdomen. They drain the GALT.
metastasis Spread of a tumor from its original location to distant organs of the body by traveling through the blood or lymphatics or by direct extension.