What is the immune system?

The immune system is the weapon of our body in front of foreign organisms. It consists in preventing them from entering our body. If they reach there, the immune system takes care to detect them and to destroy them.

The body has physical barriers against infections:


The human body presents 2 sorts of natural barriers in front of outside threats:

The first one is the skin which acts by three mechanisms:
- physical Mechanism: only a cut, a sting or a burn can allow the penetration of a foreign body. Indeed, this one is established by cells, corneocyte, performed with a hard protein, keratin, and with a lipid cement which assure the cohesion between cells and thus impermeability of skin.
- chemical Mechanism: the skin contains glands said sebaceous (sebum ) and sudoriparous (sweat) which eliminate certain bacteria.
- biological mechanism: Presence of a microbial flora which prevents the proliferation of outside substances.

The second is all the mucous membranes:
- mechanical Functioning: the wind in nostrils, the movement of eyelids and the wash of urethra by urine, prevent the bacteria from settling down.
- the mucous secretions: these fix the air particles, evacuate them and degrade them. We find also the particular antibodies there which make sluggish the foreign particles.

Detection of antigens

During an infection, the body has in addition to biological barriers, agents of defense: it is the immunizing answer!

Thus, after crossing of the natural barriers which we presented higher, the virus X activates several phenomena:
        - if it infects a target cell, this one is going to show on the surface of the plasma membrane the antigens of the virus. It is thus "marked" and the agents of defense understand that it is infected.
        - if it meets a macrophage it is ingested by phagocytosis. The macrophage then shows in the same way as the infected cell the surface antigens of the virus.

        - Finally, if it meets Lymphocyte B which wears receivers antibody corresponding to him, it stimulates it directly by contact of its surface antigens. It activates the lymphocyte which begins the process of clonal selection ( we shall speak about later).

Concerning agents of defense:

One call them lymphocytes: there are several sorts and have various roles in the immunizing answer. How form these lymphocytes, and which are their roles?

Origin of lymphocytes:


Lymphocytes result from stem cells which are in bone marrow where they proliferate and differ. We distinguish two sorts of lymphocytes, which distinguish themselves by the nature of their surface receivers which determine their functions (different genes code for a specific membrane structure in every lymphocyte). Thus, lymphocytes T differ in the thymus and lymphocytes B differ in the marrow.


What it the main role of these lymphocytes?


Answer of the defense by proliferation clonale and by differentiated mitosis


We saw that the arrival of a virus activated several phenomena:

        - The infected cells are located by a first kind of lymphocytes: LT8. They become attached to the viral antigen on the cell surface, this connection makes it sensitive to interleukine secreted by the LT4, about which we shall speak later. Beeing activated and stimulated, it multiplies and is transformed into citotoxique or killer LT8 and into memory LT8.

        - The marked macrophages are spotted by another kind of lymphocytes, the LT4. They fix to the surface antigens, are activated and differ in auxiliary LT4 who secrete the famous chemical messenger: the interleukine. They can also differ in memory LT4 (NB: this chemical messenger is a member of cytokines, soluble substances of communication acting at a distance on the other cells to regulate the activity and the function. It is not an hormone!). Auxiliary LT4 have an important role which allows to coordinate the immunizing answer and to regulate the activity of the LT8 and LB.

        - Finally, the last kind of lymphocytes, the LB. They have been activated by a direct contact with the antigen, and emit interleukine receivers. When the chemical messenger settles, the LB multiplies, they are transformed either plasmocytes which secret specific antibodies to the spotted antigen, or in LB memory.

These processes multiply the clones of lymphocytes and to differentiate them are called: the clonale selection.



Antigens elimination


All the differentiated lymphocytes are presents.
We have to eliminate the pathogenic elements.


        - the cytotoxic LT8 are going to settle on every infected cell and emit molecules which will kill it. For the more curious scientists, these molecules are perforine, a protein which produces pores in the plasmic membrane of the target cells and provoke their lysis. They also produce granzyme, a protein which can penetrate the target cell by pores caused by the perforine. They lead to the degeneration of organites and DNA: it is the apoptosis of the cell.
This mechanism plays a major role in the elimination of the infected cells and the cells abnormal as the cancer cells.
        -Plasmocytes (or white blood cells) stemming from LB are big cells with a very developed cytoplasm and can produce many antibodies. After differentiation, they work in lymph nodes and spleen. The specific antibodies in the antigens of the virus X are going to settle by complementarity on these and to form immune complexes. These will irreparably be attracted towards macrophages and will be swallowed up.

        - Then, there is a function which we left unsettles here: the important role of lymphocytes memory!
Indeed, they participate in the formation of the acquired immunity (principle on which leans the vaccine). The body already possesses an important quantity of specific LB before the infection, what allows a faster reaction. There are the LT8, LT4 and LB reports are being stored in the end of the infection on one hand in:
               -Peripheral tissues for LT, and more particularly in tissues where the first infection began.
               -In lymphoid organs for the LT and LB.

Memory LT are cells with long lasting of life. We consider that memory lymphocytes B can persist more than 50 years in a human being and this without new exhibition in their specific antigen. During a second infection of the virus X the immunizing answer is faster and more violent thanks to these memory lymphocytes !

Cleaning of cell thanks to macrophages


When pathogenic agents have been neutralized, macrophages arrive to clean wastes.

But what is a macrophage?
Macrophages are differentiated cells which finished its differentiation and thus which will not divide any more. They can stay alive several months. They stay in tissues of our body which can be affected by pathogenic agents or which secrete a lot of waste.

What is its role?
Its role is to swallow up ( destroy by a digestion process ) elements useless or harmful for our body. If there is an infection it swallows up all the immune complexes, rests of infected cells, damaged Lymphocytes, etc. … They are cleaners in the body and it avoids the accumulation of waste in the latter.
At the end, macrophages are going to locate its target and to throw repercussions towards this foreign body. In this way, it includes it, ingests it and digests it.

The pathogenic is now in the intracellular vesicle formed by the endocytosis of this one. This vesicle is going to merge with a lysosome and the destructive enzymes are going to kill and to digest the pathogenic.