Immunity and biological methods of disease prevention and control
Practically every multicellular organism, from invertebrates to vertebrates, is a potential host for various microbes. These microbes may spend some or all of their life cycle within, or upon, the bodies of their hosts. Any that gain entry to the tissues of their host may be rapidly distributed inside the host’s body by its circulatory system. A host that does not have the means to protect itself from the entry and subsequent proliferation of microbes within its body can be rap-idly overwhelmed.
The immune systems of fish and crustaceans have similarities and differences. Crustaceans are no different from other animals in that their defense system is largely based on the activities of the blood cells or hemocytes. These hemocytes, the crustacean equivalent of the vertebrate leucocytes, are capable of phagocytosis, encapsulation, nodule formation, and mediation of cytotoxic-ity. Another hallmark in crustacean immunity is the rapid sealing of wounds by blood coagulation to prevent loss of hemolymph and to immediately entrap invading microorganisms and arrest their dispersal in the body.
The fish immune system also has these non-specific mechanisms of defense but they can be differentiated from those of crustaceans as they have evolved an additional way of recognizing microbes. This recognition is the basis of what is commonly called the adaptive immune system or specific immunity. The adaptive system has two hallmarks that distinguish it from non-specific immunity. Firstly, recognition is performed by a receptor that exists in billions of different forms in an individual. This diversity endows the animal the ability to recognize any microorganism. Secondly, the adaptive system retains a memory of each particular microorganism to which it has been exposed. Memory allows the adaptive system to eliminate the same microorganism more effectively upon subsequent exposure.
The receptor that is responsible for these remarkable properties is the antigen receptor. These are found only in lymphocytes, which are found only in verte-brates. The substance or ligand, which the antigen receptor binds to, is called the antigen. Many substances that are foreign to the host can be an antigen, including proteins, polysaccharides and nucleic acids. These substances are usually components of the cell walls of microbes. There are two general catego- ries of lymphocytes based on the antigen receptor they carry: T-lymphocytes and B-lymphocytes. The binding of antigens to their receptors triggers lympho-cytes to become active in an immune response, a complex process usually re-ferred to as lymphocyte activation. An activated B lymphocyte starts to manu-facture large quantities of immunoglobulin molecules, which are then released into the blood. These soluble forms of immunoglobulin are commonly called antibodies.