Manual of basic techniques for a health laboratory - libdoc.who.int

11. Immunological andserological techniquesMany of the diagnostic techniques applied in immunology are based on the factthat antigens and antibodies interact. Most infectious diseases are diagnosed byisolating and identifying the infectious microorganism in a specimen from the patient.In some cases microorganisms are difficult to culture and isolate or may requirespecial and often expensive techniques that are not available for routinediagnosis. In other immunological disorders there is no microorganism per se toidentify or isolate. There are also some “naturally” occurring immune diseases,often classified as autoimmune diseases, which are not caused by a microorganismbut can be detected by some of the diagnostic techniques applied in immunology.Several of these techniques detect specific metabolic products or specific antibodiesand antigens. In those disease states where a microorganism is involved, theseimmunology tests do not detect the microorganism directly, but provide evidenceof its presence.A number of diagnostic techniques based on biological reactions using antigen–antibody interactions are described. It is beyond the scope of this manual to go intodetail about the immune system. The aim here is to introduce some terminologyand general concepts of the immune system, which will help in understanding someof the immunological techniques described. Note that various approaches are availablefor different diagnostic purposes; the choice should be based upon the questionasked and the availability of facilities. The diagnostic techniques describedhere are those most frequently used as an aid in the diagnosis of certain diseasesthat cause an immunological reaction.11.1 Introduction to immunologyThe role of the immune system is defence. The immune system has both nonspecificand specific mechanisms to recognize and respond accordingly to foreignand potentially pathogenic microorganisms. The non-specific defences are physicalor mechanical barriers, for example, the skin and mucous membranes. These barriersare there to prevent entry of pathogens into the body. These barriers usuallywork very well, but some pathogens do manage to enter the body where they areimmediately destroyed by phagocytic cells such as macrophages.When pathogenic microorganisms enter the body the specific defence mechanismsare activated. The specific mechanisms are divided into the humoral (antibodymediated)and the cell-mediated systems. The humoral system is associated withcells known as B-lymphocytes, which are the precursors of plasma cells. Plasmacells produce and secrete protein substances known as antibodies or immunoglobulins.The cell-mediated system is associated with T-lymphocytes whichcan process and destroy foreign bodies.11.1.1 AntibodiesAntibodies are found in serum, milk, saliva, tears, urine and other body fluids. Inneonates, antibody production is virtually absent and protection is provided bymaternal antibodies mainly through breast milk and those that cross the placenta.A growing infant is constantly exposed to various environmental antigens which328