Acquired Immunity

OVERVIEW OF ACQUIRED IMMUNITY Immunity and Susceptibility Immunity refers to the ability of an individual to recognise and defend itself against infectious diseases. Susceptibility, the opposite of immunity, is the vulnerability of the host to harm by infectious agents. Immunology and Immune System Immunology is the study of specific immunity and how the immune system responds to specific infectious agents. The immune system consists of various cells, especially lymphocytes, and organs such as thymus gland, which participate in providing the host with specific immunity to infectious agents. Types of Immune System The immune system of humans consists of two parts, humoral (antibody mediated) immune system and cell-mediated immune system, abbreviated as AMI and CMI. The humoral immune system involves antibodies that are dissolved in the blood plasma and lymph. These antibodies are produced by lymphocytes called B cells. The cellular or cell-mediated immune system depends on lymphocytes called T cells, which are located both in the blood and in the lymphoid tissues.

Specific Immunity (Acquired, Adaptive or Inducible) Acquired immunity is that immunity which a person develops during his lifetime. Acquired immunity is the surveillance mechanism that specifically recognizes foreign antigens and selectively eliminates them, and on re-encountering the antigen has an enhanced response. Six major characteristics of acquired immunity are: · Specificity · Inducibility · Diversity · Memory

. Distinguishing self from nonself . Self-limiting Humoral immunity protects against circulating extracellular antigens such as bacteria, microbial exotoxins and viruses in their extracellular phase. In active immunity the individual synthesizes his own antibodies, whereas in passive immunity the individual receives these antibodies from some other individual, either a human or a lower animal. Cell-mediated immunity (CMI). Cell-mediated immune response is a complex process which is based mainly upon thymus dependent lymphocytes called as T cells. Two sub-populations of T cells play important role in CMI: T helper (T H) cells and T cytotoxic (T d cells.Cell-mediated immunity protects against intracellular icroorganisms, such as viruses, and is important in the rejection of organ transplants and tumour cells. Actively acquired immunity: A degree of naturally acquired active immunity results from any infection from which a person recovers, whether the illness is serious or subclinical. During the illness the individual receives an antigenic stimulus which initiates antibody production against that specific pathogen. On a subsequent visitation by the same or an antigenically related pathogen, these antibodies will assist in the defense of the body. Passively acquired immunity: Naturally acquired passive immunity usually refers to the transplacental passage of antibodies from the mother to the unborn child during the later part of pregnancy. This is caused almost entirely by IgG since other immunoglobulins do not pass the placental barrier. Colostrum contains secretory IgA and IgM which are not destroyed by the poorly developed digestive system. Artificial passive immunity refers to the original production of antibodies in some other individual (either human or lower mammal) and the acquisition of these antibodies through parenteral route. Injections of hyperimmune serum (against rabies, tetanus, hepatitis, etc.) represent this type of immunization. Classification of acquired immunity Effector Type of response

Outcome of defense response

Cellular

Engulfment Cytotoxic

-Uptake of foreign material (phagocytosis) -Destruction of infected cells or nonself cells by contact

Humoral

Agglutination Precipitation Neutralization

-Clumping of organisms and phagocytosis -Clumping of soluble molecules and phagocytosis -Inactivation of toxins -Blocking of infection by viruses and other intracellular parasites

Complement

-Lysis of parasites fixation -Phagocytosis of parasites

Differences between active and passive immunity Active Character immunity Production by host's immune + system Induced by contact with + antigen

Passive immunity -

Protection induced lasts long Immunity effective after lag phase Immunological memory Used as immunoprophylaxis Used as immunotherapy Type of immunity involved Immunity inherited Useful in immunodeficient patients Duration of immunity Negative phase

+

-

+ + + CMI and humoral -

+ humoral + +

Only

+ Long May occur

Short No negative

phase

Essentials of Immune Response The essentials of immune response can be summarised in four “R”s: 1. Recognise self from nonself. 2. Respond to nonself invaders. 3. Remember the invader. 4. Regulate its action. Local Immunity: This refers to availability of immunity at the site of the entry of the pathogens. For example oral vaccines developed against various intestinal pathogens provide immunity both at the gut level and systemically. A special class of immunoglobulins called secretory antibodies (IgA) form the major component of local immunity. Herd Immunity: This refers to the overall level of immunity in a community and is relevant in the control of epidemic diseases. FUNCTIONAL ANATOMY AND DEVELOPMENT OF IMMUNE SYSTEM The immune system comprises of a variety of organs which are concerned with growth, development and deployment of lymphocytes. These are called as lymphoid organs. Lymphocytes and antigen processing cells are important constituents of non-lymphoid tissues such as those in the respiratory and gastrointestinal tracts. Functions of Lymphoid Organs are: · To provide an environment for the maturation of various lymphoid cells · To concentrate lymphocytes into organs that drain areas of antigen insult · To permit interaction between various types of lymphocytes · To efficiently disburse products of lymphocytes (antibodies or cytokines).

Types of Lymphoid Organs Primary lymphoid organs are those organs in which proliferation and differentiation of lymphocytes takes place independent of antigenic stimulation like: Thymus, Bone marrow and Intestinal epithelium. In the tissues of these organs, the evolution of stem cell progeny into mature lymphocytes takes place. The mature lymphocytes then seed into periphery, where exposure to antigen and appropriate immune responses occur. Secondary lymphoid organs are those organs which receive and maintain functional lymphocytes. These are: · Lymph nodes · Spleen · Gut or mucosa associated lymphoid tissue · Appendix · Tonsils · Adenoids Lymph nodes develop throughout the body and are most concentrated at areas where pathogens are more likely to enter. DEVELOPMENT OF IMMUNE SYSTEM The basic pattern of the immune system found in the adult animal is developed during embryogenesis. All components of the human immune system begin formation during the first three months in uterus. After about six weeks of development, the thymus, spleen and bone marrow begin to form. The last portions of the immune system to form are the lymphatic channels and lymph nodes. In humans, six lymphatic sacs of the mesodermal tissue are formed. As the foetus develops, these elongate into channels. At about 3 months masses of mesodermal cells form beside the channels, connective tissue capsule form around them, and they become vascularised. The newly developed lymph nodes sink into the channels and become populated with lymphocytes from the thymus and spleen. Lymph node development is completed within a short period after parturition.