Medical brain notes 30

  THE IMMUNE SYSTEM INTRODUCTION The  immune system  is a set of organs, tissues, cells and molecules that protect the body from disease caused by microorganisms and multicellular parasites .  Immunology is the study of the immune system and how it works. Unraveling of the mechanisms of immune defense and the use of products of the immune system in investigating and treating disease has revolutionized the biomedical sciences in a way that is comparable to that of molecular biology in recent years. TYPES OF IMMUNE DEFENSE The immune system has to distinguish between the cells and macromolecules that make up the body that is,  self , and those that are foreign or  nonself . Immunological defenses are usually classified as  nonspecific  and  specific . Nonspecific defenses constitute a first line of defense and are available immediately any foreign material, including substances such as wood splinters as well as microorganisms, enters the body. ...

  NONSPECIFIC DEFENSES Microorganisms are prevented from entering the body by structural barriers, such as the skin and the mucosal membranes. These barriers are further protected by chemical secretions, for example lactic acid in sebum and hydrochloric acid secreted in the stomach. Furthermore, mucus, secreted by the mucosal membranes that line the urogenital, respiratory and gastrointestinal tracts, contains lysozyme, an antibacterial enzyme that catalyzes the hydrolysis of the peptidoglycan wall of some types of bacteria. Should microorganisms breach these barriers, other antimicrobial proteins such as complement and interferons may be activated and/or produced. Finally, there are a number of cells that have a role in the elimination of microorganisms. These cells may be found in the blood, although a number are also found in the solid tissues. Some blood cells move between the blood and the extravascular tissues, particularly during an infection. INTERFERONS Interferons (IFNs) ...

  SPECIFIC IMMUNE RESPONSES   The specific immune response allows the development of true immunity to an infectious agent. Since true immunity can only develop after exposure to the virulent microorganism or a harmless vaccine derived from it, the response is often called  acquired immunity . For example, an individual who has had measles is unlikely to suffer that disease again, even though exposure to the virus may occur subsequently even after many years. Two major features define a specific immune response. First, specific immunity is only induced towards the agent, called the  immunogen , which stimulated it and secondly, the specific immune response is more rapid following subsequent contacts with that same immunogen. It is the rapid secondary response that prevents the development of disease in the immune individual. The ability to produce a quicker response on second contact with an immunogen is known as  immunological memory . This process is mimicked b...

  SMALL LYMPHOCYTES Small lymphocytes are the cells responsible for specific immunity ( Figure4.2 ). They make up approximately 20% of the blood leukocytes. There are twopopulations of small lymphocytes that mature at different sites in the body and have distinct functions ( Figure 4.14 ). The precursors of small lymphocytes originate in the bone marrow by division of  lymphoid stem cells . Some small lymphocytes remain in the bone marrow where they mature into  B lymphocytes . When maturation is complete, B lymphocytes have antibodies on their surface that are receptors for an individual epitope. Thus a single B lymphocyte is specific for an epitope and is capable of clonal division and of making antibody to it when stimulated appropriately by the immunogen. Thus they are responsible for humoral immunity. The second population of small lymphocytes, known as  T lymphocytes , leave the bone marrow when immature and complete theirmaturation in the fetal thymus, a bilob...

  PRODUCTION OF A SPECIFIC IMMUNE RESPONSE All small lymphocytes are specific for a single epitope on an individual immunogen. This means that they will respond only to that epitope and no other. The basis of this specificity depends on lymphocyte membrane proteins that act as receptors for individual epitopes. The receptors on B lymphocytes are surface immunoglobulins that have the same specificity as the antibody that the cells will subsequently secrete following appropriate stimulation. The surface antibodies on a single cell may belong to more than one class, so, for example, B lymphocytes may express both IgG and monomeric IgM, but these will have identical specificities. The T cell receptor (TCR) is composed of two polypeptide chains that form a single binding site. All the receptors on a single T cell have the same specificity. Although T and B cells have different types of receptor and are involved in different aspects of the immune response, there are some similarities in ...