Everything about Macrophage totally explained
Macrophages (
Greek: "big eaters", from
makros "large" +
phagein "eat") (
mø) are cells within the tissues that originate from specific
white blood cells called
monocytes. Monocytes and macrophages are
phagocytes, acting in both non-specific defense (or
innate immunity) as well as specific defence (or
cell-mediated immunity) of vertebrate animals. Their role is to
phagocytose (engulf and then digest) cellular debris and
pathogens either as stationary or mobile cells, and to stimulate
lymphocytes and other immune cells to respond to the pathogen.
Life cycle
When a
monocyte enters damaged tissue through the
endothelium of a
blood vessel (a process known as the
leukocyte adhesion cascade), it undergoes a series of changes to become a macrophage. Monocytes are attracted to a damaged site by chemical substances through
chemotaxis, triggered by a range of stimuli including damaged cells, pathogens,
histamine released by
mast cells and
basophils, and
cytokines released by macrophages already at the site. At some sites such as the testis, macrophages have been shown to populate the organ through proliferation.
Unlike short-lived neutrophils, the life span of a macrophage ranges from months to years.
Function
Phagocytosis
One important role of the macrophage is the removal of
necrotic debris and dust in the
lungs. Removing dead cell material is important in chronic inflammation as the early stages of inflammation are dominated by
neutrophil granulocytes, which are ingested by macrophages if they come of age (see
CD-31 for a description of this process.)
The removal of dust and necrotic tissue is to a greater extent handled by
fixed macrophages, which will stay at strategic locations such as the lungs, liver, neural tissue, bone, spleen and connective tissue, ingesting foreign materials such as dust and pathogens, calling upon wandering macrophages if needed.
When a macrophage ingests a pathogen, the pathogen becomes trapped in a food
vacuole, which then fuses with a
lysosome. Within the lysosome,
enzymes and toxic peroxides digest the invader. However, some bacteria, such as
Mycobacterium tuberculosis, have become resistant to these methods of digestion. Macrophages can digest more than 100 bacteria before they finally die due to their own digestive compounds.
Role in specific immunity
Macrophages are versatile cells that play many roles. As scavengers, they rid the body of worn-out cells and other debris. They are foremost among the cells that "present" antigen; a crucial role in initiating an immune response. As secretory cells, monocytes and macrophages are vital to the regulation of immune responses and the development of inflammation; they churn out an amazing array of powerful chemical substances (
monokines) including enzymes, complement proteins, and regulatory factors such as
interleukin-1. At the same time, they carry receptors for
lymphokines that allow them to be "activated" into single-minded pursuit of microbes and tumour cells.
After digesting a pathogen, a macrophage will present the
antigen (a molecule, most often a protein found on the surface of the pathogen, used by the immune system for identification) of the pathogen to a corresponding
helper T cell. The presentation is done by integrating it into the cell membrane and displaying it attached to a
MHC class II molecule, indicating to other white blood cells that the macrophage isn't a pathogen, despite having antigens on its surface.
Eventually the antigen presentation results in the production of
antibodies that attach to the antigens of pathogens, making them easier for macrophages to adhere to with their cell membrane and phagocytose. In some cases, pathogens are very resistant to adhesion by the macrophages. Coating an antigen with antibodies could be compared to coating something with
Velcro to make it stick to fuzzy surfaces.
The antigen presentation on the surface of infected macrophages (in the context of
MHC class II) in a lymph node stimulates
TH1 (type 1 helper T cells) to proliferate (mainly due to
IL-12 secretion from the macrophage). When a B-cell in the lymph node recognizes the same unprocessed surface antigen on the bacterium with its surface bound antibody, the antigen is endocytosed and processed. The processed antigen is then presented in MHCII on the surface of the B-cell. TH1 receptor that has proliferated recognizes the antigen-MHCII complex (with co-stimulatory factors-
CD40 and CD40L) and causes the B-cell to produce antibodies that help
opsonisation of the antigen so that the bacteria can be better cleared by
phagocytes.
Macrophages provide yet another line of defense against tumor cells and body cells infected with
fungus or
parasites. Once a T cell has recognized its particular antigen on the surface of an aberrant cell, the T cell becomes an activated effector cell, releasing chemical mediators known as
lymphokines that stimulate macrophages into a more aggressive form. These activated or
angry macrophages, can then engulf and digest affected cells much more readily. The angry macrophage doesn't generate a response specific for an antigen, but attacks the cells present in the local area in which it was activated.
Media
Related cells
Further Information
Get more info on 'Macrophage'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://macrophage.totallyexplained.com">Macrophage Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
