Chapter 25 - Immunology

Summary

INTRODUCTION

Immunology can be defined as the study of the reactions of a host when foreign sub stances are introduced into the body. Foreign sub stances that induce immune responses are referred to as antigens and are usually infectious agents. However, antigens may also be harm less environmental substances, such as pollen, which could trigger a response in certain individuals.

Early studies in immunology concentrated largely on the body's ability to protect itself and com bat antigens. As knowledge of the field accumulated, the scope of immunology has broadened significantly to include the study of molecular mechanisms and the genetic basis for the immune response, recognition of the body's response to self-antigens in autoimmune diseases, the phenomenon of hypersensitivity and the manipulation of the immune system to suppress graft rejection and increase immune surveillance against cancerous cells. The defence systems are divided into two broad categories: innate immunity (representing non-specific defences) and adaptive immunity (specific defences).

INNATE IMMUNITY

The innate immune system works on the basis of pattern recognition of several highly con - served structures present on or in different micro-organisms. The innate system present in all classes of plants and animals provides an immediate defence, which is not long-lasting.

The most important line of defence in the body is the physical barrier which is provided by intact skin and mucosal surfaces. The physical barrier is supplemented by:

• Continuous movement, for example the movement of cilia in the respiratory tract

• Commensal bacteria, especially in the gastrointestinal tract (GIT)

• Secretion of antimicrobial substances such as surfactant in the lungs, lysozyme in tears and hydrochloric acid in the GIT.

Once a pathogen has penetrated these physical barriers, it is recognised as foreign. This relies on certain pattern recognition molecules, such as the toll-like receptors (TLRs). These surface receptors bind to a host of diverse microbial pro ducts including components of bacterial cellular membranes such as lipopolysaccharide (LPS) from Gram-negative cell walls and peptidogly cans from Gram-positive cell walls. Receptor binding results in intracellular signalling and the release of pro-inflammatory mediators.