Immunological Reactions

In the 18th century Many people, being sure that someday in their lives they would still become infected with smallpox, deliberately exposed themselves to the possibility of infection in order to recover from this disease in more favorable conditions and not be afraid of it in the future. Even now, some parents, for the same reasons, do not protect their children from contracting childhood diseases, knowing that people get sick with some diseases only once in their lives.

This type of resistance is called acquired immunity. But a man who is immune to smallpox by having suffered from that disease is as susceptible to measles, or to any other disease, as one who has never had smallpox; therefore we say that immunity is specific. Actively acquired immunity is caused by the formation in the body of specific proteins, so-called antibodies, which are released into the blood and tissue fluids after the penetration of any foreign protein, called an antigen, into the body.

The antigen and antibody react with each other, and this protects the body from damage. If, for example, you inject a rabbit with egg albumin (a protein substance), the animal's cells respond by producing antibodies specific to this albumin. In addition, the body is capable of producing a special type of antibody, called an antitoxin, in response to the presence of a toxin (usually a protein) secreted by the bacterium.

Once a sufficient amount of antitoxin has been formed, the toxin can no longer cause harm to the body. Several decades ago it was known that antibodies formed in response to the introduction of a given antigen are not always homogeneous - they can differ in their specificity, in the degree of activity in relation to the reaction with the antigen and in physicochemical properties (size and shape of the molecule , its total charge and amino acid sequence). Antibodies circulating in the blood are associated with a certain fraction of plasma - gamma globulins.

Gamma globulins are proteins that are very similar in their physical and chemical properties, but differ in their specificity for antigens. The differences between different antibodies are quite subtle; they are even more subtle than the differences between different enzymes. Apparently, only a small part of the protein molecule (having a molecular weight of about 160,000) is immunologically active.

The differences between different antibodies seem to boil down to minor differences in the shape of the bedka molecule, in the arrangement of its constituent atoms, ensuring complementarity in the geometric configurations of the antigen and antibody, which must fit together like a key and a lock.

Lymphatic tissues usually synthesize antibodies only to “foreign” proteins, that is, to proteins that are not found in the body under normal conditions. But sometimes some normal components of the body can be antigenic and cause the formation of antibodies; As a result of the resulting antigen-antibody reaction, a person can get sick.

After the injection of the antigen, a latent period begins, lasting about a week, and then antibodies appear in the blood. The antibody titer slowly increases, reaches a low peak (primary reaction) and decreases again. A secondary injection of antigen a few days, weeks or even months later causes rapid production of antibodies after a shorter latency period (secondary reaction). The antibody titer reaches a higher level and decreases more slowly. Subsequent injections of antigen cause additional secondary reactions until the maximum titer is reached. Over time, this titer usually decreases, and periodic reimmunization helps maintain immunity at a satisfactory level. In a previously immunized person, a secondary reaction can also be caused by infecting him with a natural infectious agent; Antibodies usually form quickly enough and prevent the onset of symptoms of the disease.

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