MadSci Network: Immunology
Query:

Re: What are the specialized parts of a monocyte?

Date: Tue Nov 28 20:09:01 2006
Posted By: Cenk Sumen, Post-doc/Fellow, Immunology, Memorial Sloan-Kettering Cancer Center
Area of science: Immunology
ID: 1160932510.Im
Message:

Monocytes are white blood cells or leukocytes ("leuko" is white in Greek) that originate in the bone marrow. They can be found in the blood, but their final destination is to migrate into tissues where they differentiate into different cell types. Once monocytes migrate into tissues, they are then referred to as "macrophages" (meaning "cells that eat a lot"!) so to avoid confusion I will refer to the macrophage in the rest of this response.

Macrophages engulf a lot of particles which must be degraded to their biochemical building blocks. The membrane compartments that pinch off the surface to become internalized are called "phagosomes". Interestingly, some bacteria such as Legionella have evolved mechanisms that allow them to "escape" from these vesicles into the cytoplasm of the macrophage to avoid degradation. Phagosomes are targeted to fuse with other specialized vesicles called "lysozomes" which contain potent enzymes that can degrade proteins, sugars and other molecules.

Degradation of proteins into smaller bits called "peptides" (commonly ~5-15 amino acids long) is at the heart of adaptive immunity. In effect these small peptides can find their way back onto the macrophage surface after being "loaded" onto MHC molecules, and can stimulate T cells if they are considered "dangerous"; for example if they come from bits of processed viral or bacterial proteins during the course of an infection. Hence, macrophages contain specialized vesicles which allow them to serve their crucial role as scavangers of the immune system.


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