2ʹ-O-methyltransferase (MTase) An enzyme that transfers a methyl group to the 2ʹ hydroxyl of the first and second ribose groups in mRNA. Viruses that acquire MTase can produce cap-1 and cap-2 on their transcripts and thereby evade restriction by IFIT1.
MF-59 A proprietary adjuvant based on squaline and water used in Europe and Canada in conjunction with inﬂuenza vaccine.
MHC class I See MHC class I molecules.
MHC class I deficiency An immunodeficiency disease in which MHC class I molecules are not present on the cell surface, usually as a result of an inherited deficiency of either TAP-1 or TAP-2.
MHC class I molecules Polymorphic cell-surface proteins encoded in the MHC locus and expressed on most cells. They present antigenic peptides generated in the cytosol to CD8 T cells, and also bind the co-receptor CD8.
MHC class II See MHC class II molecules.
MHC class II compartment (MIIC) The cellular vesicles in which MHC class II molecules accumulate, encounter HLA-DM, and bind antigenic peptides, before migrating to the surface of the cell.
MHC class II deficiency A rare immunodeficiency disease in which MHC class II molecules are not present on cells as a result of various inherited defects. Patients are severely immunodeficient and have few CD4 T cells.
MHC class II molecules Polymorphic cell-surface proteins encoded in the MHC locus are expressed primarily on specialized antigen-presenting cells. They present antigenic peptides derived from internalized extracellular pathogens to CD4 T cells and also bind the co-receptor CD4.
MHC class II transactivator (CIITA) Protein that activates transcription of MHC class II genes. Defects in the CIITA gene are one cause of MHC class II deficiency.
MHC haplotype A set of alleles in the MHC that is inherited unchanged (that is, without recombination) from one parent.
MHC molecules Highly polymorphic cell-surface proteins encoded by MHC class I and MHC class II genes involved in presentation of peptide antigens to T cells. They are also known as histocompatibility antigens.
MHC restriction The fact that a peptide antigen can only be recognized by a given T cell if it is bound to a particular self MHC molecule. MHC restriction is a consequence of events that occur during T-cell development.
MIC-A, MIC-B   MHC class Ib proteins that are induced by stress, infection, or transformation in many cell types and are recognized by NKG2D.
microautophagy The continuous internalization of the cytosol into the vesicular system.
microbial glycolipids Diverse class of antigens frequently presented by CD1 molecules to iNKT cells.
microbiome See commensal microorganisms.
microbiota See commensal microorganisms.
microclusters Assemblies of small numbers of T-cell receptors that may be involved in the initiation of T-cell receptor activation by antigen in naive T cells.
microfold cells See M cells.
microglial cells An embryonically derived form of tissue macrophage in the central nervous system that is dependent on IL-34 for local self-renewal throughout life.
minor histocompatibility antigens Peptides of polymorphic cellular proteins bound to MHC molecules that can lead to graft rejection when they are recognized by T cells.
minor lymphocyte stimulating (Mls) antigens An old term referring to non-MHC antigens responsible for unusually strong T cell responses to cells from different strains of mice, now known to be superantigens encoded by endogenous retroviruses.
mismatch repair A type of DNA repair that causes mutations and is involved in somatic hypermutation and class switching in B cells.
missing self Refers to the loss of cell-surface molecules that engage with inhibitory receptors on NK cells, resulting in NK-cell activation.