Antigenic variation is the ability of a pathogen to evade the immune responses of the host using various methods, often interfering in some way with production, action or communication of the cells of the immune system
The trypomastigotes the extracellular form of T.cruzi, are carried in the blood entering via the bite or mucosal membranes, including the conjunctiva, the trypomastigote then enters cells in the bite area where it differentiates into the intracellular amastigotes. The amastigotes multiply via binary fission, transform into the trypomastigote and are released back into the bloodstream where they are transported to new tissue sites (the majority being muscle tissue, but many cell types can become infected) re-infecting cells and beginning the cycle again.
A cell-mediated response involves the cell itself and presentation of the pathogen's antigen fragments, by the MHC class 1 molecules on the cell surface, which are recognised by cytotoxic T cells; the cell is then signalled to die preventing spread of infection. A humoral response involves B-cells, which recognise intact antigens in solution and on the surface of infected cells via their surface antibodies, differentiating into plasma cells that secrete antibody.
Cytokines are protein signalling molecules that take part in many processs and are part of many mechanisms in the body. They are important in the immune system for carrying information about pathogen antigens from cell to cell, allowing the cells to be destroyed. Matching receptor molecules are found on the cell surface, often only after the cell has been activated. Interlukins 2-26 are molecules involved in signalling between leukocytes. Interferons-alpha, beta and gamma help to protect cells against intracellular pathogens.
Over time pathogens evolve ways of evading and combatting the immune responses of the host. Several mechanisms are used.
Antigenic disguise is a process whereby the the pathogen integrates host proteins on to its cell surface which enables the pathogenic cell to produce host proteins, this process also inactivates the lytic pathway. (more)
Many pathogens produce their own proteins that have evolved to counteract immune effector mechanisms such as the synthesis of proteins that inhibit lysome fusion
Some pathogens produce proteins that trap antibodies so they cannot opsonise target cells for macrophage destruction.
Antigenic variation is also used by some pathogens. (see above)
An intracellular pathogen is one which invades the cell directly often using the cells mechanisms for its own use-as is the case with viruses which have no replication machinery of their own and therefore cannot survive very long outside a host cell. Some bacteria and some protoctista are intracellular pathogens. Extracellular pathogens invade the host but remain ouside the cell often invading the tissues, the mucus membranes or the other systems of the body.
Macrophages are long-lived immune cells with powerful pathogen killing mechanisms. Pathogenic material is engulfed by the macrophage before destruction, this is used to the advantage of some intracellular pathogens as a way to enter cells.
Virulence factors are those factors, which enable the pathogen to cause damage and disease in host tissues. Pathogen products such as toxins (produced by some bacteria such as the botulism toxin) or any protein produced that interferes with the host cell mechanisms, and example is inactivation of the lytic pathway.
Chagas' disease exhibits an acute stage often without symptoms. Enlarged lymph nodes, fever and swelling of the eyelids are classic indications of the disease. The chronic stage can exhibit after two months, and is subdivided into three catagories, intermediate; no signs of internal damage (60-70% of chronic patients), the cardiac form exhibits varying degrees of internal danage (30-40% of patients), and the digestive form exhibiting oesophageal narrowing and associated symptoms (10% of patients)
The Southern Cone programme uses vector control methods such as spraying of domestic and animal buildings with pyrethroid insecticides to kill the triatomine bugs; serological bloos screening to eleminate infection from blood transfusion; continuous surveillance of triatomines preventing re-infestation and public education programmes providing people at risk with information regarding hygiene, housing improvement and health education. The initiative of the Central American Countries and the Andean Countries Initiatives are similar programmes to the Southern Cone Programme initiated at a later date. Countries at risk with no active surveillance or control methods include, USA, Gyana, Surinam and French gyana.
Argentina, Brazil, Chili, Paraguay and Uruguay have now attained transmission free status in either the whole of the country or in parts.
DNA vaccines can be produced in bulk and are easy to store in different forms, either dried or in solution, without the need for cold storage for transportation and preservation, the cost is drastically reduced in poorer remote areas in developing countries.