Immunity
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Immune reaction

     In reaction to a “foreign body”, the antigen, the immune system defends our body by producing several antibodies. A vaccine simulates the disease, that is to say that it presents an harmless antigen, against which the body learns to defend itself. Then, the body will benefit, in advance, from a reserve of antibodies, in case the real disease breaks out. The vaccine is just stimulating our immune mechanisms, to face future viruses.

    Since Pasteur (1822-1895), the initiator of vaccine, prevention of viral infection has made great progress. But it was an English doctor named Jenner (1749-1823) who was the first to practice vaccination without even realizing it: he introduced the smallpox microbe, coming from the udder of a cow, to one of his patients on whom the effect was mild, and noted that the patient did not contract the disease when injected with the pus of a smallpox pustule. The name vaccine comes from the Latin word vacca meaning, "cow". Pasteur, for his part, demonstrated that a microbe or a toxin can become harmless while retaining their quality as triggers of an immune response.

     In the case of the non-specific immune reaction: the reaction is general, regardless of the type of microorganism that enters the human body.

    Brigitte Solère (1997) defines this phenomenon as the set of cellular and molecular events which allow the body to recognize a foreign element, destroy it and keep an immunological memory of this reaction. This means that upon first contact with a foreign body, the immune system triggers a primary immune reaction. Even if this reaction is not fast enough to quickly remove the pathogenic element and therefore avoid the disease. However, it ensures healing and remembers the defense system used. During a second contact with the same element, a rapid immune reaction is created which this time quickly destroys the foreign element, thanks to the memory of the first contact. The secondary immune reaction then makes it possible to prevent a disease from being caused by the pathogen.

- when the pathogens are extracellular (bacteria, toxins), they are destroyed by the immune system using antibodies located in the blood and lymph. This process is called a humorally mediated immune reaction.

- when the pathogens are intracellular - these are mainly viruses - the immune system destroys them using cytotoxic lymphocytes. This is called a cell-mediated immune reaction.

    The humoral immune reaction consists of two phases. In the first phase, B-lymphocytes (B for “bone”) are stimulated by the presence of the antigen. The antigens are proteins on the surface of lymphocytes that acts as a specific receptor for antigen recognition. The lymphocytes then produce other proteins, the antibodies which will attack the antigen, encompass it and thus slow down the infection. The second step involves macrophages (large cells that eat other dead cells and their debris) and T-lymphocytes (T for the "thymus" where they mature). Helper T-lymphocytes are activated by contact with macrophages using a specific receptor. The macrophage is called an antigen-presenting cell. The lymphocytes then form a helper T-cell clone that is adapted to the specific antigen. The T-cells then stimulate the B-cells, which have already encountered that particular antigen..

    So, B cells and macrophages represent antigen-presenting cells, but there is a difference. Macrophages can present different antigens while B lymphocytes only present one type of antigen. Thus, in the case of the reaction to the hepatitis A virus, antibodies responsible for fighting the hepatitis A virus (abbreviated anti-HAV) consisting of IgM (immunoglobulin M) are present in the serum. The humoral immune response to virus infection is defined by these IgMs. There are IgA antibodies that neutralize the virus in the intestine. IgG antibodies are very numerous and their life expectancy is much longer; they also ensure the protection of the body against probable reinfection.

    There are one antigen and two antibodies against the antigen corresponding to hepatitis A. These are:

- the AgHAV antigen: throughout the week preceding jaundice, there is viremia and elimination of the virus in the stools;

- the AcHAV IgM antibody: it occurs at the first clinical signs and disappears from the blood one to three months later; these two points mean that it is acute hepatitis;

- the AcHAV IgG antibody: it appears after the AcHAV IgM antibody and persists for life, indicating whether the person has had hepatitis A or not. It indicates recent or old hepatitis.

    The cell-mediated immune response consists of two phases:

- the first is similar to the selection of LT4 lymphocytes carried out during the humoral-mediated reaction, but the macrophages absorb the foreign element and remove the epitope which they incorporate into their membrane. Then they head to a place rich in LT. Only LT4s that have the epitope-specific receptor stick to the macrophage.

- the second signal is the selection and stimulation of LT8 lymphocytes (that are also selected by a double selection). They recognize the antigen, thanks to their membrane receptors.