Which are the main features of the immune system?

1.- The capability of discriminating between its own and foreign components.

The immune system has the capability of discriminating between self and foreign components, by reacting against everything different to itself (Antigens). The immune system has the amazing characteristic of reacting against any foreign molecule, different to its own structure, no matter how small this molecule is. However, it does not react against its own structures. This characteristic is one of the most important principles of immunology. During the embryonic stage, the lymphocytes that recognize their own structures are eliminated, by means of an apoptosis process, (programmed cell death). Only the cell clones that react against foreign structures will pass into circulation, whenever they are associated to the own SLA, as well as the clones tolerant to the self structures. The histocompatibility antigens (SLA) play a main role in this selection. Sometimes, the immune system may be mistaken when discerning self and foreign components. Thus, it is possible that the immune system does not react against a foreign particle. This phenomenon is called tolerance. On the other hand, the immune system can react against its own structures. These reactions are called autoimmunity.

In the thymus, lymphocytes are selected in such a way that they can only react against the foreign particles (positive selection). Only these lymphocytes will go into blood circulation. In addition,, cellular destruction (apoptosis) of the lymphocytes that react against the self structures is produced.

2.- Specificity.

The specificity of the immune system is due to the fact that both lymphocytes and antibodies only recognize one epitope or antigenic determinant. The immune system can recognize thousands of millions of different antigens, but for each determinant, a specific lymphocyte will be induced. There are as many stimulated lymphocytes as determinants forming the antigen.

Specific recognition of a single antigenic determinant (one determinant = one cell). After the recognition, a proliferation of cells takes place.Because of this the lymphocytes become part of the memory lymphocytes, and others will act as effector cells.

3.- Memory

When an antigen is first presented to the immune system, a primary response is produced. Memory lymphocytes remain for each of the antigen epitopes. When this antigen again comes into contact with the immune system (secondary response), the memory lymphocytes are stimulated in order to produce as many clones of specific lymphocytes as are needed against this particular epitope.This occurs in a faster and more effective way than in the primary response.