Immune cells
Monocytes
Monocytes, unlike granulocytes, are long lived and can differentiate to become tissue resident macrophages and dendritic cells. Cells of the monocytic lineage play an essential role in initiating and maintaining immune responses by acting as antigen-presenting cells. Monocytes are covered more fully under the section of “immunology”.
Granulocytes
The granulocytes are classified as neutrophils, eosinophils, or basophils on the basis of cellular morphology and cytoplasmic staining characterisits. These cells are covered more extensively under “immunology”.
Other Associated Immune Cells
Mast cells are formed in the bone marrow during and released into the blood as undifferentiated precursor cells and do not differentiate until they enter the tissues. They can be found in the skin and mucosal epithelial tissues of the respiratory, genitourinary and digestive tracts. Like basophils, they have large numbers of cytoplasmic granules containing histamine and play an important role in the development of allergies.
M cells are specialized cells that transport antigens from the lumina for the respiratory digestive and urogenital tracts to underlying .
NKT cells are a trace population of T cells sharing some features of NK cells, which express an invariant TCR?-chain and respond to glycolipid Ags presented on CD1d. Autologus tissues provide self-glycolipid ligands such that NKT cells are in a chronic state of low level activaiton with suppressor funciton mediated by IL-4. Tumors like melanoma provide glycolipid ligands like GD3.
The presentaiton of glycolipid ligand by Cd1d-expressing DCs or the simultaneous delivery of a microbial stimulus to DCs resulting in high IL-12p70 secretion leads in either case to robust activation of NKT cells and IFNy scertion. The critical oeprative is the need for DCs to convert NKT cells from suppressor to effector function.
NKT cells rearrange TcR genes and express a CD3/TcR receptor. NK cells do not do this.
Immune Cells of the Nervous System
Microglia cells:
Microglia are the immune cells of the central nervous system and consequently play important roles in brain infections and inflammation. Recent in vivo imaging studies have revealed that in the resting healthy brain, microglia are highly dynamic, moving constantly to actively survey the brain parenchyma. Microglia constitute 10-15% of the CNS. Hortega. Bol de la Soc esp de biol. 1919; 2. Ransohoff. Science. 2016; 3. Reu et al. Cell Reports. 2017
A critical difference between peripheral iimmune cells and microglia is the longevity of the microglial cells relative to peripheral immune cells. A couple of studies suing C14 dating as well as relevant ouse models estijate that ost of the human cortical microglial population is renewed throughout life at an average rate of 28% per year. Since microglia are long lived tissue resident macrophages with a life span of over several years, it is possible that normal aging resutls in a gradual loss of microglial function in the CNS resulting in microglial senescence. Genome-Wide Associattion studies from large European and American consortia identified common genetic polymorphisms at loci harboring gnes with microglia-specific epxrewssion pattern in the CNS, such as ABC7, CD33 and members of the MS4A genes cluster as genetic determinants of AD risk. (Rhinn, “Shifting paradigms: The central role of microglia in Alzheimer’s disease” Neurobiology of Disease 143 (2020))