The innate and adaptive immune systems provide p protection against an array of i infectious organ isms that vary in size, method of entry into the body, tropisms, reproduction, and pathologies. Fortunately, the immune system has a built-in redundancy that activates several different types of responses against a particular invader. As long as at least one of these responses is effective, the infection can be eliminated or controlled.<br />A. Humoral responses<br />The innate humoral responses are preexisting and begin acting against infectious agents upon initial contact. Activation of the complement system, through either the mannan-binding lectin pathway or the alternative pathway, provides an almost immediate and highly effective barrier to microbial growth and reproduction. <br /> l . Responses to I nfectious Agents immune mechanisms. C3b and C4b act as opsonins to accelerate the phagocytic ingestion and destruction of microbes, whereas C3a,C4a, and C5a help to initiate inflammation by attracting and activating leukocytes. With the subsequent involvement of the adaptive immune response, antibodies augment the role of complement by initiating the classical pathway. Antibodies also tag infectious agents for destruction by phagocytes (opsonization) or by natural killer cells and eosinophils (antibody-dependent cell-med iated cytotoxicity) . lgE can trigger the release of inflammatory mediators by mast cells and basophils, an important element in i m munologic resistance to parasitic worms. Finally, antibodies can inhibit entry of microbes into the body and into cells by neutralization. The effectiveness of humoral responses against infectious organisms varies, depending on the localization and specific structural features of each organism .<br />B. Cell-mediated responses<br />Many infectious agents not only enter the body, but once inside, go on to enter individual cells. Some may be taken up by phagocytes using toll-like receptors or Fe receptors and complement receptors. However, many microbes, such as viruses and some bacteria, facilitate their own entry into host cells as part of their natural life cycle. Once inside, they are sheltered from the actions of antibodies and complement, and cell-mediated responses are required to clear the infection .With in cells, viruses persist and reproduce in the cytosol. Most intracellular bacteria live with in endosomes formed during their entry into the cells. The cytosolic and endosomal compartment, however, are not completely isolated from one another. Extracellular viruses (and cellular debris containing viral particles) can be taken up by endosomes via phagocytosis. In addition , some intracellular bacteria can exit endosomes and enter the cytosol , as can some of their products or fragments. Regardless of the route taken, the localization of the infectious organism/material in the cytosolic or endosomal compartments determines the type of cell-mediated response they elicit. Infectious <br />organisms or products i n the cytosol will be processed and presented on MHC class I molecules, eliciting responses by CD8+ T cells. Infectious organisms and fragments present in endosomes (phagolysosomes) will be processed there and presented by M H C class II molecu les, generating CD4 + T cel l responses. The cytotoxic T-cel l responses (CD8 +) and delayed (-type) hypersensitivity (CD4+) responses that are generated are then capable of destroying the infected cells, disrupting the reproduction of the invasive organisms, and destroying the remaining microbes.<br /><br /><br />MSc Ola Abdullah Mahdi