5.3 The lymphoid or immune system (LAD): a network of lymphocytes and antibodies.

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Evolution of an animal species (phylogenesis) is a learning process on the time-scale of aeons. What is usually called 'learning' occurs during the lifetime of one individual (ontogenesis). Learning by the immune system consists in selecting the most appropriate antibody response to an environmental challenge. Acquiring (learning) the best fitting immune response to a new antigen, occurs in a time-scale of days/weeks.

Map 5.2.3: The lymphoid system for adaptation and defence

Map 5.2.3 is phylogenetically arranged: components of the immune system that have been formed early in the evolutionary history are close to the centre of the sphere. The core systems are surrounded by ancillary subsystems and these are resistant to change. Humans share the same subsystems with many other vertebrate species. Closer to the surface, in interaction with the environment, are highly differentiated components of the system: the antibodies. They are phylogenetically younger and more adjustable. Several types of immune-globulins (antibodies) that are active in man, are not found in the lower vertebrates. The map also shows some aspects of ontogeny: at birth only the central part of the sphere is fully developed. The more peripheral zones mature and differentiate in the course of a learning process, that takes place in blood-forming organs, thymus and lymph-nodes during early childhood. (The relation of growth, maturation and learning is explained in Map 9.5.1)

The lymphoid or immune system is a refined system for chemical recognition of the surrounding world. Its function can be defined as supplying the young individual with an up to date internal image of the environment in which it is growing up. Since every individual has it's own specific history of encounters, each resulting image differs from anyone else's. Relevant stimuli from the environment are called antigens or immunogens, since they give rise to an immune response, i.e. secretion of antibodies. Every meeting with an antigen is recorded and stored as a memory trace in the network of lymphocytes and the proteins they produce. Antigenic properties are also labelled in a value system that discriminates between self and non-self, and this label is attached to the record. When confronted later, and in another context with a familiar antigen, the system will decide if the stimulus is in the self or non-self category and whether it is acceptable (to be approached) or unacceptable and to be avoided. It does so by continuously checking antigens against the individual's identity-tags: checks are carried out to see if the component can be assimilated or should be rejected.

Most of the relevant information is coded in the form of surface configurations on antibodies, i.e. macromolecular proteins belonging to the class of immune globulins (Ig's). They circulate freely, or remain on the surface of the lymphocytes that produce them. Lymphocytes (white blood cells) of various types are produced in bone marrow. Their rate of production is facilitated or inhibited, depending on the antigenic properties of the surface structures which

they carry with them. When a lymphocyte finds a good match with a relevant antigen it's production is selectively reinforced: the bond between stimulus and receptor is the signal which starts reproduction. Antibodies of various types also recognize each other, as if they were themselves antigens. Thus they facilitate or inhibit the growth rate of their respective parent lymphocytes. As a result, a network is set up and maintained, in which oscillating population densities keep each other in check. The wave-propagation of population-increase and -decrease is similar to the one described in 3.1 to 3.3.

N.K. Jerne (1985) has compared the endless combinatory possibilities of meaningful configurations on the surface of antibody molecules with words and sentences of a language. Just as the LAD generates an appropriate response to molecules that the system has never met before, so the VAD (language) can formulate, in a novel combination of syllables and phrases, meaningful expressions that have never before been used. This analogy between language and the immune system drapes the LAD with a poët's cloak.

Summing up the function of the lymphoid system, one can say that the lymphocyte network keeps its image of the material world up to date. It educates itself continually by scanning the internal and external environment for familiar and unfamiliar features. The information is processed (evaluated and labelled) and passed on to the deeper structures for permanent storage. A schematic representation of components of the lymphoid system is given in Map 5.2.3 They are arranged in spherical form around the genome. The standard or yard-stick for compatibility of antigens is a complex of features in the genome: human lymphocytic antigen complex (HLA) or, in more general terms, the major histocompatibility complex or MHC. This is "given" and does not change in the individual's life time. It can only be changed in future generations by selective mating of the individuals who carry the genome.

Less rigid and more adaptable are other parts of the lymphoid system situated closer to the periphery of the sphere. They form networks as described, and are in immediate contact with the environment, performing such executive tasks as scanning, recognizing and tagging antigens for rejection or assimilation.

5.4 Conditions for acquiring biological immunity.