Evolutionarily, the innate immune system evolved or developed before the adaptive immune response and appears to be inherent in all multicellular organisms. One function that differentiates the innate and adaptive immune systems will be the possession of germ-line-encoded receptors discovered within the innate immune system.
These germ-line-encoded receptors mediate innate immune recognition. These receptors are predetermined genetically and thus have evolved via natural selection to possess specificity against infectious microorganisms. The immune system in higher animals and humans evolved with increasing complexity in terms of its specific antibody and cell-mediated immune response capacity, i.e., the adaptive immune response. Despite the complexity of the human immune system, host defense is still highly dependent on surface barriers and phagocytic mechanisms.
The absence of phagocytic functions (as in severe neutropenia or in neutrophil function disorders, such as chronic granulomatous illness) or the loss of physical barriers (as with extensive cutaneous burns) can present a risk of fulminant, life-threatening invasion by microorganisms that normally are not pathogenic.
In mature mammals, the main lymphoid organs are the thymus and bone marrow. During fetal development the liver is one of the main organs of lymphoid improvement. In early fetal development, lymphocyte precursors are derived from the fetal yolk sac. By the fourth or fifth week of gestation, lymphocytes originate from the liver and thereafter from the bone marrow.
In the bone marrow, pluripotent stem cells differentiate into lymphocytes, granulocytes, monocytes, erythrocytes, and megakaryocytes. B cells undergo early growth in the bone marrow and finally emerge with membrane-bound surface IgM or both IgM and IgD, even though they have not yet encountered antigen. This growth that takes location in the bone marrow is antigen-independent B-lymphocyte maturation.
B cells also proliferate in response to antigen-dependent signals and eventually differentiate into antibody-secreting cells or plasma cells. This proliferation is dependent on antigen binding towards the B-cell receptor. This receptor comprises membrane-bound immunoglobulin and two additional chains required for its stable expression.
You will find two such heterodimers that flank the membrane-bound immunoglobulin and help to mediate signal transduction. B-cell activation also demands costimulation in the form of T cell help in two ways. The interaction of these molecules facilitates class switching to IgA, IgG, and IgE. In a uncommon clinical immunodeficiency state, patients with X-linked hyper-IgM syndrome lack CD40L and are unable to produce antibodies of the IgA, IgG, and IgE classes. B cells also possess a cell surface molecule, B7 (CD80), which is up-regulated following ligation of CD40. B7 is a counterreceptor for CD28, a costimulatory molecule expressed on T cells. This costimulatory molecular interaction optimizes cytokine secretion and the cell interaction.
A human fetus is capable of synthesizing IgM antibody by 10.5 weeks of gestation, IgG by 12 weeks, and IgA antibody by 30 weeks. The immunocompetent human infant, usually born without antigen stimulation (unless infected in utero), has little circulating IgA and IgM. IgG antibody in the newborn is almost totally derived from the mother by active and selective transport across the placenta.
Adult serum levels of IgG, IgM, and IgA are attained at different developmental stages. From 6 to 8 weeks of gestation, T-lymphocyte precursors migrate via the thymus, which is derived from the third and fourth embryonic pharyngeal pouches and is situated in the mediastinum.
The thymus functions to create T lymphocytes and will be the website of initiation of T-lymphocyte differentiation. A large quantity of T cells migrate to the thymus and become fully immunocompetent T cells. In addition, numerous T cells that are autoreactive die in the thymus. Under the influence of numerous cytokines (such as thymosin), T cells in the thymus undergo growth and differentiation and deletion of autoreactive clones. Functional improvement of cellular responses progressively matures as the fetus develops through parturition and infancy into adulthood.
Phagocytic cells are seen in the human fetus at 2 months of gestation as a couple of myelocytes and histiocytes present in the early yolk sac stage of hematopoiesis. Monocytes first seem in the spleen and lymph nodes at 4 to 5 months of gestation, with gradual maturation of macrophage function with advanced fetal age. The spleen, lymph nodes, and gut-associated lymphoid tissue are considered secondary lymphoid organs. Lymph nodes are peripherally dispersed throughout the physique and function to localize the spread of infection. Lymph nodes are arranged in a reticular pattern with a cortex and medulla.
B-lymphocytes are discovered in the cortex (follicles and germinal centers) as well as in the medulla, whereas T lymphocytes are primarily discovered in the medullary and cortical areas of the lymph nodes. The spleen is also divided into T- and B-cell areas comparable to that of the lymph node. The spleen functions primarily to filter and procedure antigens from the blood.
Complement components are synthesized by the fetus early in gestation, either in the exact same time as or just before the beginning of immunoglobulin synthesis. There's nearly no placental transfer of complement components C1q, C2, C4, C3, and C5, and the total hemolytic complement in the newborn is low. Such deficiency and dysfunction may be responsible for the relative opsonic deficiency in newborns. Complement plays a very important role in both innate and adaptive immunity.
In the humoral immune response, complement opsonizes antigen as well as immune complexes for uptake by the complement receptor kind 2 (CR2, CD21), is really a coreceptor for B-cell activation, and is expressed primarily on B cells, follicular dendritic cells (FDC), and some T cells. Many various mechanisms are responsible for the complement-mediated promotion of the humoral immune response. These include:
C3d is the ligand for CR2 on B cells, and is instrumental in B cell activation. CR2 can play a role in the improvement of autoimmune disease by determining B cell tolerance toward self-antigens. CR2 may be a key element in the observed correlation in between autoimmune disease and deficiency of the early complement components.
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