1.1 The Immune Defence

The Immune Defence
The human immune system is an association of organs, cells and messenger substances distributed throughout the body which cooperate with one another in fascinating ways. Only by working together can they fulfil their major duty: protecting the individual from innumerable pathogens which are always present everywhere in his surroundings, and eliminating deviate or unusable cells within the body. Without the immune system, the complicated organism we call a human being would not be able to survive.

Coli bacteria / Itch mite / Mumps virus


The Cellular Defence
The total weight of the immune system comes to about 2 kilograms. Just under 1.5 kilograms of that is made up of white blood cells. These comrades in the ongoing battle for human health are also called leukocytes. The name "blood cells" is somewhat misleading, for they are found everywhere in the intercellular spaces of the body tissues and in the lymph vessels, not only in the blood.

Phagocyte / T cell / Leukocytes
/ The Humoral Defence = Non-cellular Defence
The leukocytes are not left to their own devices to fight intruders. They are supported by soluble components of the blood such as the antibodies. Antibodies have a sophisticated mechanism for recognizing, hindering and marking intruders so that the leukocyte troops can find them.
Antibody (model)

Messenger Substances and Toxins
A complex network of chemical processes has been spun around the leukocytes and the antibodies. In particular, the messenger substances and toxins aid the troops in their work. Thanks to the help of the messenger substances, the defence cells can communicate with one another. Some of the defence cells employ toxins to kill pathogens or infected cells.

Reactions to the Immune Defence
The immune defence is constantly in action. Normally we do not notice all of this hustle and bustle - unless the immune system has to fight serious infections. Then we are smitten with the symptoms of the raging battle, e.g., fever, weakness, nausea, swelling, reddening, nasal congestion, coughing. The resources of the entire body are then deployed to support the defensive battle. Rest, warmth, vitamin-rich nourishment and possibly medications can help the immune system to render the pathogens harmless.

The Immunological Memory
Pathogens which have already been beaten once have virtually no chance to attack successfully a second time. The immune system has reserves of memory cells. If a person is re-infected by the same pathogen, the defence normally responds so quickly that the attackers are neutralized immediately and the body never displays any symptoms of illness. /
B-memory cell

The immunological memory can also be trained to react to a particular pathogen by employing a protective vaccination before exposure to an infection without making the person seriously ill.

1.2 Protective Barriers


Macrophoto of the skin / The Skin
People owe a special debt of thanks to their outer "protective wall," the skin, for keeping the frequency of infection as low as it is. The skin is colonized by microorganisms which are the body's allies. They like the relatively "acidic" environment of the skin (pH value 4-6) and crowd out other, dangerous germs. As long as the skin is adequately covered with body-friendly microorganisms, harmful germs are unable to gain a foothold. This is one reason why it is important that the "acid protective coating" and the oily film on the skin not be destroyed by aggressive cleaning agents. Our important allies would be "washed away."

The skin is vulnerable in other ways. An injury, a cut, a puncture or a scrape exposes the sensitive body tissue lying underneath. Then pathogens can enter the bloodstream directly.

Mucous Membranes
Body orifices such as the mouth, nose, eye, ear, intestine and genitals are lined with mucous membranes which are kept constantly moist with secretions. Pathogens literally become coated in slime and are excreted along with the substance.

Many germs are taken in with food. Secretions in the mouth and stomach help to fight off pathogens. Saliva, for example, contains enzymes which attack bacteria, and stomach acid dissolves most of the undesired intruders.

Germs friendly to the body settle in the mucous membranes of the intestine. This intestinal flora keeps many harmful bacteria and fungi at bay with its secretions.
The protection provided by the mucous membranes is limited, especially when the number of pathogens is so great that they overwhelm the defence. Moreover, a number of pathogens are also capable of attacking or even penetrating the mucous membranes. /
Macrophoto of the intestinal
mucous membrane
If the cold virus, for example, succeeds in surmounting the slime barrier, it attaches itself to the mucous membrane cells, penetrates them and then multiplies inside. A host of cold viruses is quickly formed which can be eliminated only if the immune system acts against them specifically.

1.3 Defensive Action by the Immune System

The immune defence has three tasks: localization, identification and destruction of intruders. If a pathogen has overcome protective barriers such as the skin or the mucous membranes, it runs into specialized Defensive Actors.

The Patrols of the Immune System: Phagocytes = Macrophages
The phagocytes belong to the vanguard of the immune defence. They devour more or less everything which appears foreign to them. Since they attack all possible pathogens indiscriminately, they are regarded as representatives of the non-specific defence. The phagocytes are responsible for keeping bacteria, viruses or fungi under control especially during the early phase of an infection. If the non-specific defence succeeds in eliminating all of the pathogens, the infection has been defeated. No immunological memory is formed. /
Phagocyte (model)

Animated
film / If, however, the non-specific defence cannot deal with the invasion of pathogens by itself, the phagocytes send out a call for additional troops. They break the pathogens down into pieces and present parts of them on their surface as a kind of "wanted" poster. This wanted poster, such as a fragment of the outer shell of a bacteria, is also called an antigen.

The Alarm System of the Phagocytes
Phagocytes employ a special technique to display antigens on their surface. They use transport molecules of the types MHC-I and MHC-II.

The abbreviation MHC stands for major histocompatibility complex. All of the body's cells bear MHC-I molecules on their surface. In a manner of speaking, the MHC molecules serve as identification papers. They vary from one individual to the next and help the immune system to distinguish between own and foreign. MHC-II molecules appear only on antigen displaying cells.

MHC molecules have a kind of depression in which the antigen is presented as if on a display platter. This is the only way to ensure that other cells of the immune system become aware of the pathogen and initiate appropriate defence actions. Phagocytes can also call up support by sending out messenger substances.

The Scouts and Commanders of the Immune System: T Helper Cells
The immune system has at its disposal a gigantic army of scouts, the T helper cells. They can ferret out the antigens, the typical properties of pathogens. And as "commanders" they coordinate important sections of the defence to fend off an infection. T helper cells are a part of the specific immune defence.

Antigens can appear in any imaginable form. There is a matching T helper cell for every possible antigen, just to make sure that not a single antigen is overlooked. So there is strict division of labour: every T helper cell recognizes only one specific antigen.

The Recognition Mechanism of the T Helper Cells
Every T helper cell bears a large number of receptors, all of the same type, all around its body. Using these receptors, the T helper cell probes the surface of other cells and determines if the antigen which its receptor matches is located there.

The receptor of the T helper cell basically consists of two sub-receptors, the antigen receptor and the so-called CD4 receptor.
·  The antigen receptor matches a specific antigen.
·  The CD4 receptor fits the MHC-II molecule. /
T helper cell (model)

A T helper cell recognizes an antigen only if both sub-receptors can simultaneously dock onto a phagocyte or another cell presenting an antigen. If only the antigen receptor matches the surface antigen of a cell, the T helper cell is deactivated. So T helper cells can react only to antigens which are displayed to them by the body's own antigen-presenting cells. The MHC-II molecules are, so to speak, a kind of pass.

The Activation of a T Helper Cell
If a T helper cell is to initiate defensive action against a pathogen, it must first be activated by a phagocyte. The phagocyte makes use of the co-stimulator, a molecular "ignition key," to start the T cell.

The phagocyte releases a flood of messenger substances with the objective of raising a powerful immune defence. The T helper cells then divide several times, producing an armada of T helper cells with the same properties. They swarm out to mobilize reinforcements in the fight against the pathogen.

The Weapons Factory of the Immune System: B-Cells
The T helper cells look specifically for B cells which, with the help of the MHC-II molecule, present the targeted antigen and can manufacture the corresponding antibodies. If a T helper cell can dock on with its receptors, it releases messenger substances. The B cells multiply, and "weapons factories" appear which produce large quantities of antibodies. /
B cell (model)
Animated
film
/ The immune system has at its disposal a huge number of B cells differing from one another so that the corresponding antibody can be prepared for every single antigen. There is strict division of labour here as well: every B cell can produce only a single type of antibody. B cells are a part of the specific defence.

Antibodies (model) / The Guided Weapons of the Immune System: Antibodies
Antibodies are designed in the shape of a "Y". The two arms bear identical receptors which match a specific antigen exactly. Antibodies are a part of the specific, humoral defence.

The antibodies search for pathogens which bear the corresponding antigen on their surface and attach themselves. This has two effects.

·  The pathogens enveloped by the antibodies (= antigen-antibody complexes) are immobilized and can no longer threaten the body's own cells.

·  The pathogens are marked as hostile by the antibodies and can be destroyed by the phagocytes.

Although antibodies differ from one another in their receptors, their "stems" are all the same (Fc region). Phagocytes recognize the "stems," latch on to them, and devour the pathogen together with the antibodies. A kind of rubbish pick-up, the phagocytes gradually eliminate the enemy pathogens.

The Immunological Memory
When the immune system has successfully vanquished an infection, the majority of the combatants (T helper cells, B cells) have fallen as casualties. But some of them remain in reserve as memory cells. If infection by a pathogen which has already been defeated once recurs, the T- and B-memory cells are quickly reactivated, and production of appropriate antibodies can be running at top speed again in only a brief time. So the immune defence is prepared for the infection: the body is immune to the pathogen. /
Animated
film

1.4 Active and Passive Immunization

Although the immune system is basically equipped to deal with infections of any kind, the struggle against the pathogens does not always run smoothly. If a person's defences have been weakened, an infection can result in serious health impairment, or even be fatal.
While the non-specific immune defence battles pathogens immediately, it takes 7-10 days until a specific immune defence can be set up. During this period of time, bacteria, viruses or parasites can achieve such strength that the specific immune defence is simply too late. This can be prevented in many cases by a preventive vaccination. /
Prof. Reinhard Kurth,
Paul-Ehrlich-Institute:
Statement on vaccines
/ Active Immunization
Active immunization for a particular pathogen is based on a brilliant deceptive manoeuvre. Instead of using a dangerous pathogen, the vaccine contains a weakened strain which the immune system cannot distinguish from the original. The advantage: the administration of a small quantity of weakened pathogens is normally harmless for the person being vaccinated, but still results in the desired formation of memory cells. They are now standing by on call and can fight the dangerous original pathogen at any time without any noticeable delay.
B cell produces antibodies
Vaccines
Modern vaccines are extraordinarily safe; undesirable side effects are extremely rare. But a certain risk of damage from the vaccination cannot be excluded completely. And overly sensitive people may have an allergic reaction.
Physicians basically use two types of vaccines for the immunization: living vaccines and deactivated vaccines.
·  Living vaccines contain living bacteria or viruses which are able to reproduce, but have been greatly weakened in their ability to produce a disease. Living vaccines are considered to be especially effective. However, there is an extremely slight risk that the vaccination will cause illness. One possibility is that the weakened pathogen may, during reproduction, convert back to the strain which causes the illness; another is that the immune system of the vaccinated person has previously been damaged.·
·  Deactivated vaccines contain bacteria or viruses which have been killed, or elements of these germs. So there is no reproduction of the pathogen in the body. There is practically no risk of becoming ill from deactivated vaccines. /
Reproduction of yellow fever virus in chicken eggs

Vaccination
Deactivated vaccines consist only of parts of the shell or of antigens of the pathogen. Whatever may be hidden behind this "wanted" poster, the immune defence now has its "enemy" and reacts exactly as if it were having to deal with the original strain of the pathogen which causes the disease. This is why it basically makes no difference if a vaccine contains weakened or dead pathogens.

Vaccination Reactions
When the body's immune system reacts to a vaccine, symptoms may appear for a short time: e.g.