The Immune response is the body's ability to stay safe by affording protection against harmful agents and involves lines of defense against most microbes as well as specialized and highly specific response to a particular offender. This immune response classifies as either innate which is non-specific and adaptive acquired which is highly specific. The innate response, often our first line of defense against anything foreign, defends the body against a pathogen in a similar fashion at all times. These natural mechanisms include the skin barrier, saliva, tears, various cytokines, complement proteins, lysozyme, bacterial flora, and numerous cells including neutrophils, basophils, eosinophils, monocytes, macrophages, reticuloendothelial system, natural killer cells (NK cells), epithelial cells, endothelial cells, red blood cells, and platelets.
The adaptive acquired immune response will utilize the ability of specific lymphocytes and their products (immunoglobulins, and cytokines) to generate a response against the invading microbes and its typical features are:
The inflammatory immune response is an example of innate immunity as it blocks the entry of invading pathogens through the skin, respiratory or gastrointestinal tract. If pathogens can breach the epithelial surfaces, they encounter macrophages in the subepithelial tissues that will not only attempt to engulf them but also produce cytokines to amplify the inflammatory response.
Active immunity results from the immune system's response to an antigen and therefore is acquired. Immunity resulting from the transfer of immune cells or antibodies from an immunized individual is passive immunity.
The immune system has evolved for the maintenance of homeostasis, as it can discriminate between foreign antigens and self; however, when this specificity is affected an autoimmune reaction or disease develops.
While the immune system is meant to protect the individual against threats, at times an exaggerated immune response generates reaction against self-antigens leading to autoimmunity. Also, the immune system is not able to defend against all threats at all times.
Cells of the innate immunity are:
Cells of the adaptive response are:
Mesoderm cells are induced to form hemangioblasts, a common precursor for vessels and blood cell formation. The definitive hematopoietic stem cells derive from mesoderm surrounding the aorta called aorta-gonad-mesonephros region. These stem cells colonize the liver and after that are actively produced by the bone marrow by the seventh month of gestation.
The organ systems involved in the immune response are primarily lymphoid organs which include, spleen, thymus, bone marrow, lymph nodes, tonsils, and liver. The lymphoid organ system classifies according to the following:
T lymphocytes mature in the thymus, where these cells reach a stage of functional competence while B lymphocytes mature in the bone marrow the site of generation of all circulating blood cells. Excessive release of cytokines stimulated by these organisms can cause tissue damage, such as endotoxin shock syndrome.
The immune system responds variedly to different microorganisms often determined by the features of the microorganism. These are some different ways in which the immune system acts
Immune response to bacteria - Response often depends on the pathogenicity of the bacteria:
Immune response to fungi:
Immune response to viruses:
Immune response to parasites:
Despite Immune response(s) generated by intact and functional Immune system we still fall sick, and this is often due to evasive mechanisms employed by these microbes. Here are some of those.
Strategies of viruses to evade the immune system
Antigenic variation: It is a mutation in proteins that are typically recognized by antibodies and lymphocytes. HIV continually mutates, thus making it difficult for either the immune system to protect against it and also hinders the development of a vaccine.
By disrupting 2',5'-oligoadenylate synthetase activity or by the production of soluble interferon receptors viruses disrupt the Interferon response.
By several mechanisms, Viruses affects the expression of MHC molecules.
A virus can infect immune cells: Normal T and B cells are also sites of virus persistence. HIV hides in CD4+T cells and EBV in B cells.
Strategies of bacteria to evade the immune system
Intracellular pathogens may hide in cells: Bacteria can live inside metabolically damaged host leukocytes, and escaping from phagolysosomes (Shigella spp).
Strategies of fungi to evade the immune system
Strategies of parasites to evade the immune system
The most important mechanisms of the immune system by which it generates immune response include:
Macrophages produce lysosomal enzymes and reactive oxygen species to eliminate the ingested pathogens. These cells produce cytokines that attract other leukocytes to the site of infection to protect the body. The innate response to viruses includes the synthesis and release of interferons and activation of natural killer cells that recognizes and destroys the virus-infected cells. The innate immunity against bacterial consist of the activation of neutrophils that ingest pathogens and the movement of monocytes to the inflamed tissue where it becomes in macrophages. They can engulf, and process the antigen and then present it to a group of specialized cells of the acquired immune response. Eosinophils protect against parasitic infections by releasing the content of their granules.
Antibody-dependent cell-mediated cytotoxicity (ADCC): A cytotoxic reaction in which Fc-receptor expressing killer cells recognize target cells via specific antibodies.
Affinity maturation: The increase in average antibody affinity mostly seen during a secondary immune response.
Complement system: It is a molecular cascade of serum proteins involved in the control of inflammation, lytic attack on cell membranes, and activation of phagocytes. The system can undergo activation by interaction with IgG or IgM (classical pathway) or by involving factors B, D, H, P, I and C3, which interact closely with an activator surface to generate an alternative pathway C3 convertase.
Anergy: It is the failure to induce an immune response following stimulation with a potential immunogen.
Antigen processing: Conversion of an antigen into a form that can be recognized by lymphocytes. It is the initial stimulus for the generation of an immune response.
Antigen presentation: It is a process in which specific cells of the immune system express antigenic peptides in their cell membrane along with alleles of the major histocompatibility complex (MHC) which is recognizable by lymphocytes.
Apoptosis: Programmed cell death involving nuclear fragmentation and the formation of apoptotic bodies.
Chemotaxis: Migration of cells in response to concentration gradients of chemotactic factors.
Hypersensitivity reaction: A robust immune response that causes tissue damage more considerable than the one caused by an antigen or pathogen that induced the response. For instance, allergic bronchial asthma and systemic lupus erythematosus are an example of type I and type III hypersensitivity reactions respectively.
Inflammation: Certain reactions that attract cells and molecules of the immune system to the site of infection or damage. It featured increased blood supply, vascular permeability and increased transendothelial migration of blood cells (leukocytes).
Opsonization: A process of facilitated phagocytosis by deposition of opsonins (IgG and C3b) on the antigen.
Phagocytosis: The process by which cells (e.g., macrophages and dendritic cells) take up or engulf an antigenic material or microbe and enclose it within a phagosome in the cytoplasm.
Immunological tolerance: A state of specific of specific immunological unresponsiveness.
The immunological investigations for the study of innate and adaptive immunity are listed below and include the assessment of immunoglobulins, B and T-lymphocyte counts, lymphocyte stimulation assays, quantification of components of the complement system and phagocytic activity.
Quantitative Serum Immunoglobulins
IgG antibodies (post-immunization)
IgG antibodies (post-exposure)
Detection of isohemagglutinins (IgM)
Blood lymphocyte subpopulations
Lymphocyte stimulation assays
Nitroblue tetrazolium (NBT) test (before and after stimulation with endotoxin)
Complement System Evaluation
Measurement of individuals components by immunoprecipitation tests, ELISA, or Western blotting
Complement system functional studies
Measurement of complement-activating agents
Assays for complement-binding
Others complement assays
The immune system protects the body against many diseases including recurrent infections, allergy, tumor, and autoimmunity. The consequences of an altered immunity will manifest in the development of many immunological disorders some of which are listed below:
Highly specific and discriminatory immunity is of utmost importance for survival. The immune system has evolved as a collection of protective mechanisms to defend the host against a long list of potential invaders that would take advantage in immunodeficiency disorders, inflammatory diseases, cancers, and autoimmunity. This system has to be sophisticated enough to recognize "self" from "non-self" and provide help in infections, malignant tumors, organ transplantations, and various other situations the immune system encounters.
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