Immunology Terms Starting With M

Macrophage is a large, long-lived phagocytic cell of the innate immune system derived from blood monocytes or embryonic precursors that engulfs and destroys pathogens, dead cells, and debris while also orchestrating inflammatory and tissue-repair responses.

Macrophages reside in virtually every tissue, where they acquire distinct names and specialized functions: Kupffer cells in the liver filter bacteria from portal blood, microglia in the brain survey for neural damage, and alveolar macrophages in the lungs clear inhaled particles and pathogens. A single macrophage can phagocytose dozens of apoptotic cells per hour through a process called efferocytosis, preventing the release of inflammatory intracellular contents that would otherwise damage surrounding tissue. Macrophages also present antigens on MHC class II molecules to CD4 T cells, linking innate detection to adaptive immune activation.

Depending on the cytokine environment, macrophages polarize toward classically activated states that kill pathogens or alternatively activated states that promote tissue repair, a distinction first formalized by Siamon Gordon and colleagues in the 1990s.

Major Histocompatibility Complex is a densely packed genomic region encoding cell surface glycoproteins that present peptide antigens to T cells, with class I molecules displayed on all nucleated cells and class II molecules restricted to professional antigen-presenting cells.

MHC class I molecules present peptides of 8 to 10 amino acids derived from intracellular proteins to CD8 cytotoxic T cells, allowing the immune system to detect and destroy virus-infected or cancerous cells. Class II molecules present longer peptides from phagocytosed extracellular material to CD4 helper T cells, directing responses against bacteria and parasites that have been engulfed by phagocytes. The MHC region in humans, called the human leukocyte antigen (HLA) complex and located on chromosome 6, is the most polymorphic region of the human genome, with more than 30,000 allelic variants catalogued across its loci as of 2023.

This extraordinary diversity means that unrelated individuals rarely share identical HLA profiles, which is why matching donor and recipient HLA types is critical for successful organ and stem cell transplantation.

Mast Cell is a long-lived tissue-resident innate immune cell derived from bone marrow progenitors that stores preformed mediators including histamine and heparin in cytoplasmic granules and synthesizes prostaglandins and leukotrienes upon activation, driving allergic and inflammatory responses.

Mast cells mature in peripheral tissues including skin, gut mucosa, and lungs, where they are primed by binding IgE antibodies to high-affinity Fc?RI receptors on their surface. Crosslinking of receptor-bound IgE by allergen triggers rapid degranulation within seconds, releasing histamine, proteases, and heparin, followed by de novo synthesis of eicosanoids that sustain inflammation over hours. Beyond allergy, mast cells contribute to innate defense against bacteria, parasites, and venoms through rapid mediator release and cytokine production.

Each mast cell can contain up to 1,000 granules, making it one of the most heavily armed resident immune cells in peripheral tissue.

Memory Cell is a long-lived lymphocyte generated during a primary immune response that persists after antigen clearance and mediates faster and stronger secondary responses upon re-exposure to the same antigen.

Memory B cells produce higher-affinity antibodies with switched isotypes upon reactivation, reflecting the somatic hypermutation and class switching undergone in germinal centers during the primary response. These T cell counterparts differ from naive T cells in their lower activation threshold, more rapid effector function, altered homing receptors that guide them to peripheral tissues, and reduced dependence on co-stimulation. The duration of immunological memory varies considerably: measles immunity persists for life in most individuals, while influenza memory wanes over years as the virus mutates its target epitopes.

Studies of smallpox vaccination recipients found circulating memory B cells and T cells more than 50 years after immunization, confirming that some memory populations are extraordinarily durable.

MHCII is a cell surface glycoprotein expressed on professional antigen-presenting cells that presents peptides derived from extracellular or phagocytosed proteins to CD4 helper T cells, initiating adaptive immune responses against pathogens and other foreign antigens.

MHC class II molecules consist of non-covalently associated alpha and beta chains encoded by the HLA-DR, HLA-DQ, and HLA-DP gene clusters in humans. Peptides of 13 to 25 amino acids are loaded into the MHC class II groove in late endosomes after proteolytic processing, assisted by the HLA-DM chaperone that exchanges low-affinity peptides for higher-affinity ones before the complex reaches the cell surface. MHC class II expression is constitutive on dendritic cells, macrophages, and B cells, but IFN-gamma can induce its expression on many other cell types, expanding antigen presentation capacity during inflammation.

Polymorphism at HLA-DR, HLA-DQ, and HLA-DP loci is among the strongest genetic risk factor for autoimmune diseases including type 1 diabetes and rheumatoid arthritis, reflecting how peptide-binding specificity shapes self-tolerance.

Monocyte is a large circulating white blood cell of the myeloid lineage that patrols the bloodstream, responds rapidly to infection signals, and differentiates into macrophages or dendritic cells upon entering tissues.

Monocytes constitute three to eight percent of circulating white blood cells in humans and are divided into classical, intermediate, and non-classical subsets with distinct functions and surface markers. Classical monocytes, identified by high CD14 and low CD16 expression, are recruited to sites of infection and inflammation by chemokines including MCP-1, where they differentiate into tissue macrophages or inflammatory dendritic cells in response to local signals. Non-classical monocytes patrol the vascular endothelium and survey for damage, removing cellular debris and responding to nucleic acid signals through TLR7 and TLR8.

During homeostasis, circulating monocytes also seed resident tissue macrophage populations in the gut and lung, replenishing cells lost to normal turnover.