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Macrophages ( magnified 250,000 times through an electron microscope ) constitute just one battalion in the remarkable
army our bodies marshall in our defense against a teaming host of bacterial and viral invaders that constantly threaten to
overcome us. We call this unseen army the immune sytem. Against most ordinary foes,such as cold viruses and minor bacterial
infections,the immune system is a good protector.
Pitted against more insidious foes,like cancer and AIDs,its's often overwhelmed. And,in some conditions,known as autoimmune
diseases, the immune system turns against us and attacks our own health cells,which is what happens in such forms of arthritis
as lupus and rheumatoid arthritis amongst others.
Blood is the battlefield where the immune system plies its trade. Pulsing through the average adult's circulatory system
( a complex network of viens, arteries, and capillaries ) are about five-and-a-half quarts of red tide - "the vital force
of life" . Our understanding of the blood's intricate role in our affairs ( among other things,it transports nutrients and
hormonal messengers, hauls waste,clots wounds,and fights viral and bacterial infections ) only really began with the invention
of the microscope in the 16 th century.
Known to scientists as erythrocytes,these tiny cells constitute about 45 % of the blood. Their job is to haul oxygen
from the lungs to each of the body's 60 trillion cells via an iron-containing protein called hemoglobin, which binds to oxygen
molecules released from the lungs ( a process called oxgenation ) to form a loose,unstable compound called oxyhemoglobin.
Whenever one of these oxygenerated red cells encounters another of the body's cells that "gasping for air ",it releases
its oxygen in exchange for the depleted cell's carbon dioxide,which it then carries back to the lungs to be exhaled.
Red cells,though,don't have any direct involvement with the immune system;that's the province of white blood cells (
leukocytes) ,600 times fewer than their red kin. There are 5 types:Eosinophils and basophils,neutrofils and monocytes,and
lymphocytes.
Eosinophils and basophils are key players in inflammatory and allergic reactions,while neutrophils and monocytes are
constantly engaged in phagocytosis ( its root means "too eat" ),an apt description of how these cells dine on foreign invaders,dead
cells,and the daily accumulation of other biological debris.
Billions of these phagocytes (cell eaters ) are produced in the bone marrow every day,from whence they travel through
the circulatory system.the body's tissues and organs,engulfing and breaking down captured materials.
Neutrophils are relatively short-lived. Not so monocytes triggered by chemicals exuded by abacteria or dying cells (chemotaxis
),monocytes puff like fat men at a feast,balloning to 10 times their former size to become lurching macrophages,literally
"big eaters" .
They live for years,and in the face of an otherwise unmanageable foe, they can fuse together to form giant cells or swell
their numbers by reproducing.
Lymphocytes,comprising about 25 % of all white blood cells,are the front-line soldiers in the immune system. Unlike
other white blood cells that "crawl" through the blood stream,most lymphocytes ( which are derived from parent cells in bone
marrow called stem cells ) take up residence in body tissues,especially lymphatic tissue
From here,things get complicated,but a deeper understanding of the immune system will help a more active role in our
treatment.
An antigen is any substance,usually a bacterium or a virus,that "turns on" the immune response. This response ( its often
referred to as "the immune cascade" ) consists of two main components: humoral immunity and cell-mediated immunity.
To understand the difference, we have to go back to lymphocytes,of which there are two basic types, B cells,which mature
in bone marrow,and T cells,which undergo further differentiation in the thymus,a small gland directly beneath the breastbone.
B cells are the agents of humoral immunity; T cells are the agents of cell-mediated immunity.
Both systems can be activated by the presence of antigens. In humoral immunity,B cells produce one of the body's most
potent weapons against invaders,protein antibodies composed of gamma globulins,also known as immunoglobulins. Antibodies are
target specific;i.e.,each type binds to and destroys only one specific antigen.
For example,an antibody that attacks enterococci bacteria will ignore a flu virus,and vice versa. In fact,the 10 trillion
B cells we each cart about in our blood are capable of fabricating more than 100 million distinct antibodies at any one time,each
composed of four tiny protein chains.
They form Y-shaped sites ( two to an antibody ) expressed on the cell membrane as receptor molecules. When a receptor
recognizes its antigen,the enemy it was specifically created in the bone marrow to attack,the antibody binds to the antigen,causing
the B cell to proliferate in a "clonal-selection process" that produces a variety of deadly progeny,both more B cells and
free-wheeling antibodies.
These proteins don't destroy foreign organismisms themselves;they merely "mark" the invader chemically for destruction
by other elements of the immune system
One of the most potent of these is complement,a brigade of enzymes that fracture in response to the chemical signals
that are emitted when an antibody binds to an antigen. The fragments thus created also bind to the antigen in a precise sequence;once
the last one is in place,the enzyme act like chemical dynamite and literally blow the antigen up.
In addition to attracting complement,antibody-antigen groupings,called immune complexes,draw any nearby macrophages,which
require little prompting to dig in a big meal. Immune complexes can also bind with other antigens or proteins or even other
antibodies ( a process called agglutination ),serving larger dishes to patrolling macrophages and neutrophils.
Cell-mediated immunity can be roused into action by B cell activity which divide and differentiate into plasma cells
that secreate antibody proteins,soluable forms of their receptors that bind to antigens,either neutralization them or precipitating
their destruction by scavenging cells,such as macrophages. ( The response of phagocytes to foreign invaders is a non-specific
immunity,they eat any material they encounter .)
Macrophages and B cells activate T cells in a similar fashion: They break the antigen they've engulfed into tiny fragments
( called antigenic peptides ),which are joined to molecules called major histocompatibility complex ( MHC-a group of
closely linked genes ) and transported to the surface of the cell,where they're placed on display,so to speak.
T cells have receptor molecules on their surfaces that are capable of recognizing portions of different antigenic peptide-MHC
combinations. When they do,they leap into action,producing lymphokines,chemical messengers that stir up other elements
of the immune system, including more B cells and macrophages.
T lymphocytes are if anything even more important to the immune response thean B cells. Even if all the body's B lymphocytes
were healthy and active,without T cells an effective immune respose would be impossible.
Unlike B cells there are three types of T cells,whose only apparent difference is function; The first,known as CD-8-killer,or
"cytotoxic"-cells bind directly into the antigen,inflicting a fatal wound upon it. Exactly how it knocks off its victims isn't
clear,but the process is probably closer to the enzyme action of complement than the devouring and destroy technique favoured
by the macrophages.
The second type CD-4,or helper cells are activated by an antigen to stimulate the production of vast numbers of lymphokines
that in turn accelerte the production of vast numbers of lymphokines that in turn accelerate the production of other elements,such
as B cells and other T cells specific to the triggering antigen,thus promoting inflammation.
T helper cells may be the big guns in the immune system,since even one helper cell can activate hundreds of B cells and
macrophages by releasing its lymphokines ( which include various interleukins, interferon -gamma,tumor necrosis factor,or
TNF,colony stimulating factor, lymphotoxin, and T-cell replacement factor ).
One type of lymphokine activates B cells and macrophages;another behaves as a "migration inhibition factor" that orders
lymphocytes to remain on the job. Others kill adjacent infected cells or broaden the immune response of lymphocytes to
include different target antigen. by contrast,suppresor T cells,as their name suggests,have the opposite effect-damping down
B and T cells.
A select number of both B and T cells remain in the blood long after the invader has been destroyed to serve as "memory"
cells: lymphocytes that will quickly mount an attack ( in 4 to 5 days ) on any subsequent invasion by the same antigen. These
memory cells are the basis of "bestowed" immunity that results from an infection or vaccination.
All this activity characterizes the immune system when its working properly.
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