Alloimmunity
Alloimmunity (sometimes called isoimmunity) is an immune response to nonself antigens from members of the same species, which are called alloantigens or isoantigens. Two major types of alloantigens are blood group antigens and histocompatibility antigens. In alloimmunity, the body creates antibodies (called alloantibodies) against the alloantigens, attacking transfused blood, allotransplanted tissue, and even the fetus in some cases. Alloimmune (isoimmune) response results in graft rejection, which is manifested as deterioration or complete loss of graft function. In contrast, autoimmunity is an immune response to the self's own antigens. (The "allo-" prefix means "other", whereas the "auto-" prefix means  self.) Alloimmunization (isoimmunization) is the process of becoming alloimmune, that is, developing the relevant antibodies for the first time. Alloimmunity is caused by the difference between products of highly polymorphic genes, primarily genes of the major histocompatibility complex, of the donor and graft recipient. These products are recognized by T-lymphocytes and other mononuclear leukocytes which infiltrate the graft and damage it. Types of the rejection. Transfusion reaction. Blood transfusion can result in alloantibodies reacting towards the transfused cells, resulting in a transfusion reaction. Even with standard blood compatibility testing, there is a risk of reaction against human blood group  systems other than ABO and Rh. Hemolytic disease of the fetus and newborn. Hemolytic disease of the fetus and newborn is similar to a transfusion reaction in that the mother's antibodies cannot tolerate the fetus's antigens, which happens when the immune tolerance of pregnancy is impaired. In many instances the maternal immune system attacks the fetal blood cells, resulting in fetal anemia. HDN ranges from mild to severe. Severe cases require intrauterine transfusions or early delivery to survive, while mild cases may only require phototherapy at birth. Transplant rejection. Acute rejection. Acute rejection is caused by antigen -specific Th1 and cytotoxic T-lymphocytes. They recognize transplanted tissue because of expression of alloantigens. A transplant is rejected during first several days or weeks after transplantation. Hyperacute and accelerated rejection. Hyperacute and accelerated rejection is antibody-mediated immune response to the allograft. Recipient's blood already contains circulating antibodies before the transplantation – either IgM or antibodies incurred by previous immunization (e.g. by repeated blood transfusion). In case of hyperacute rejection, antibodies activate complement; moreover, the reaction can be enhanced by neutrophils. This type of rejection is very fast, the graft is rejected in a few minutes or hours after  the transplantation. Accelerated rejection leads to phagocyte and NK cell activation (not of the complement) through their Fc receptors that bind Fc parts of antibodies. Graft rejection occurs within 3 to 5 days. This type of rejection is a typical response to xenotransplants. Chronic rejection. Chronic rejection is not yet fully understood, but it is known that it is associated with alloantibody and cytokine production. Endothelium of the blood vessels is being damaged, therefore the graft is not sufficiently supplied with blood and is replaced with fibrous tissue (fibrosis). It takes two months at least to reject the graft in  this way. Mechanisms of rejection. CD4 and CD8 T-lymphocytes along with other mononuclear leukocytes (their exact function regarding the topic is not known) participate in the rejection. B-lymphocytes, NK cells and cytokines also play a role in it. B-lymphocytes. Humoral (antibody-mediated) type of rejection is caused by recipient's B-lymphocytes which produce alloantibodies against donor MHC class I and II molecules. These alloantibodies can activate the complement – this leads to target cell lysis. Alternatively, donor cells are coated with alloantibodies that initiate phagocytosis through Fc receptors of mononuclear leukocytes. Mechanism of humoral rejection is relevant  for hyperacute, accelerated and chronic rejection. Alloimmunity can be also regulated by neonatal B cells. Cytokines. Cytokine microenvironment where CD4 T-lymphocytes recognize alloantigens significantly influences polarization of the immune response. NK cells. NK cells can also directly target the transplanted tissue. It depends on the balance of activating and inhibitory NK cell receptors and on their ligands expressed by the graft. Receptors of KIR (Killer-cell immunoglobulin-like receptor) family bind concrete MHC class I molecules. If the graft has these ligands on its surface, NK cell cannot be activated (KIR receptors provide inhibitory signal). So if these  ligands are missing, there is no inhibitory signal and NK cell becomes activated. It recognizes target cells by “missing-self strategy” and induces their apoptosis by enzymes perforin and granzymes released from its cytotoxic granules. Alloreactive NK cells also secrete proinflammatory cytokines IFN-γ and TNF-α to increase expression of MHC molecules and costimulatory receptors on the surface of APCs (antigen-presenting cells). This promotes APC maturation which leads to amplification of T-cell alloreactivity by means of direct and also indirect pathway of alloantigen recognition (as described below). NK cells are able to kill Foxp3 regulatory T-lymphocytes  as well and shift the immune response from graft tolerance toward its rejection. Besides the ability of NK cells to influence APC maturation and T cell development, they can probably reduce or even prevent alloimmune response to transplanted tissue – either by killing the Donor APCs or by anti-inflammatory cytokine IL-10 and TGF-β secretion. However it is important to note that NK cell sub-populations differ in alloreactivity rate and in their immunomodulatory potential. Concerning immunosuppressive drugs, the effects on NK cells are milder in comparison to T cells. T-lymphocytes. Alloantigen recognition Alloantigen on APC surface can be  recognized by recipient's T-lymphocytes through two different pathways: Activation of T-lymphocytes T-lymphocytes are fully activated under two conditions: Alloimmune response can be enhanced by proinflammatory cytokines and by CD4 T-lymphocytes that are responsible for APC maturation and IL-2 production. IL-2 is crucial for memory CD8 T cell development. These cells may represent a serious problem after the transplantation. As the effect of being exposed to various infections in the past, antigen-specific T-lymphocytes have developed in patient's body. Part of them is kept in organism as memory cells and these cells could be  a reason for “cross-reactivity” – immune response against unrelated but similar graft alloantigens. This immune response is called secondary and is faster, more efficient and more robust. Graft tolerance. Transplanted tissue is accepted by immunocompetent recipient if it is functional in the absence of immunosuppressive drugs and without histologic signs of rejection. Host can accept another graft from the same donor but reject graft from different donor. Graft acceptance depends on the balance of proinflammatory Th1, Th17 lymphocytes and anti-inflammatory regulatory T cells. This is influenced by cytokine microenvironment, as mentioned before, where CD4 T-lymphocytes are activated and  also by inflammation level (because pathogens invading organism activate the immune system to various degrees and causing proinflammatory cytokine secretion, therefore they support the rejection). Immunosuppressive drugs are used to suppress the immune response, but the effect is not specific. Therefore, organism can be affected by the infection much more easily. The goal of the future therapies is to suppress the alloimmune response specifically to prevent these risks. The tolerance could be achieved by elimination of most or all alloreactive T cells and by influencing alloreactive effector-regulatory T-lymphocytes ratio in favor of regulatory cells which could inhibit alloreactive  effector cells. Another method would be based on costimulatory signal blockade during alloreactive T-lymphocytes activation.