The immune system

All higher animals have the ability to recognize foreign and potentially harmful molecules entering their bodies. Efforts are made to isolate or expel such foreign molecules. The immune system is the major defence mechanism against substances that have gained entry. A substance capable of exciting such a reaction from the immune system is called an "antigen". The body's reaction to the recognition of this antigen is to manufacture a protein called an antibody. An antibody, recognizing an antigen, links to it by a series of chemical bonds. These bonds are individually very weak (non-covalent) but their number overcomes this weakness. The locking of an antibody and its antigen is rather like the linking of two large pieces in a jigsaw puzzle. The combination of the two molecules sets in motion a series of events within the body which may in the elimination of the antigen from the body.

Antibodies are proteins that circulate in the blood and are also called immunoglobulins. They are divided into five main classes on the basis of their physical characteristics, such as molecular weight, and are referred to by a letter associated with each class, i.e., IgG, IgA, IgM, IgD, and IgE (Table 1.1). Some subclasses have specific physiological functions. IgE is associated with the immune reaction of allergic responses and IgA is the major immunoglobulin component of external body secretions such as tears or saliva. IgG is the commonest immunoglobulin found in the circulating plasma and accounts for 75% of immunoglobulins.

Table 1.1

Class	Heavy Chains	Molecular Weight	Relative Serum Concentration (%)
IgG		150,000	75
IgM		900,000	8
IgA		160,000	16
IgD		180,000	<1
IgE		180,000	<1

As well as producing antibodies, the immune system can also respond through its cellular components. This cellular immunity is largely responsible for the rejection of organ transplants, delayed allergic responses and graft-versus-host reactions. A classical example of cellular immunity is the response of tuberculin, the coat protein of the tubercle bacillus, when injected under the skin. The characteristic red zone of edema that appears after 48 hours is due to local infiltration of activated lymphocytes around the injected protein. Experiments in mice have shown that the development of cellular immunity is dependent on the presence of the thymus in early life. The thymus is a gland in the upper chest that shrinks after childhood. Lymphocytes conditioned in the thymus are referred to as T-lymphocytes.

Humoral immunity is conferred by the presence of circulating immunoglobulins in tissue fluids. In birds, the lymphocytes that produce circulating antibodies are dependent upon the presence of an organ known as a Bursa of the Fabricius, associated with the large intestine. Although there is no know comparable organ in man, such antibody-producing B-lymphocytes are made immortal in the laboratory, and allowed to continue to synthesize antibodies.