Immune System

The immune system probably evolved primarily as a defense against infection by omnipresent pathogenic organisms present in the environment.

According to Campbell (1990), "Animal's body is linked to a citadel under constant attack from without and in danger of treacherous assaults from within, while the immune system is the armed force ready to mobilize for defense at a moment's notice." So in the absence of an effective immune system, massive infections leading to death are inevitable

There are two general categories of defense mechanisms; nonspecific and specific. Collectively they both are responsible for maintaining the animal's internal milui free of microorganisms. Non specific defenses, which help prevent the entry of harmful microorganisms, include the anatomic barrier, phagocytes and natural killer cells, the inflammatory response and antimicrobial proteins. Anatomical barriers, the body's first line of defense consist of physical and chemical obstacles to the entry of intruders. The skin's outer layer (keratin) is nearly impregnable barrier of cells through which most pathogenic organism cannot penetrate. This physical barrier is reinforced by chemical defenses such as tears, saliva, gastric juices and secretions of sweat glands. The Mucus membrane and cilia of the respiratory tract also help keep microorganisms away from the epidermal surface. Microorganisms that may penetrate the outer barrier and enter the underlying connective tissue are confronted by phagocytic cells such as neutrophils and macrophages and non-phagocytic cells such as natural killer cells. Neutrophils and macrophages attack and destroy microorganisms by engulfing them, whereas natural killer cells destroy the body's own cells that have been infected by virus and can reproduce only within the host cells. Inflammatory responses increase blood flow and leakiness of the capillaries that enhance the migration of granulocytes (neutrophils, eosinophils and basophils) from blood to interstitial fluid, where they engulf microorganisms. Antimicrobial proteins function by attacking microorganisms directly or preventing their reproduction (Johnson, 1983; Curtis, 1983; Campbell, 1990)

 Specific Defense Mechanisms

Specific immune responses differ from nonspecific in their degree of specificity. They are specific reactions of the body against "non-self' foreign agents, in which the immune product reacts specifically with the stimulating agent Three essential characteristics of immune response are: 1) Recognition of foreign molecules as distinct from "self' tissue. The substances so recognized are called antigens, which stimulate the proliferation of cells that either attack the invader directly or produce defensive proteins called antibodies. 2) Antibodies and the defensive cells produced against an antigen are very specific regarding their action. They attack only the substance against which they were produced and are ineffective against any other foreign substances. 3) Besides its specificity, the immune system has ability to remember antigens it has encountered before and responds more rapidly, more strongly and more lastingly on subsequent exposure (Barrett, 1983; Curtis, 1983and Campbell, 1990).

Primary and Secondary Immune Responses

A newly introduced antigen evokes a primary immune response characterized by a long lag between the stimulus and the original detection of antigens. This lag period is associated with the time it takes the lymphocytes to differentiate into effector B and T cells that are capable of combat against the antigen. In a primary immune response, antibody levels rise exponentially; reach a steady state after some days and then decline

 Secondary Immune Responses

If a previously immunized animal is re-exposed to the immunizing antigen, an enhanced secondary immune response is elicited. The secondary immune response is characterized by a shorter lag period, higher rate and longer persistence of antibody synthesis, higher antibody titer and increased affinity, avidity and cross reactivity of antibodies

The ability to mount a secondary immune response can persist for many years due to the existence of long-lived memory cells. However, immunological memory is induced only by T-dependent antigens and no memory exists for T-independent antigens (Virella, 1993a). IgM is the major class of antibodies produced during primary immune response, whereas IgG antibodies are predominant over IgM in the secondary immune response