Immune System response to virusDepending on the nature of the infectious agent, the immune response will use the resources more effective. In viral infections develop different mechanisms, both in front of the free virus, and against the infected cell.
The viruses need the cellular mechanisms of their cells they infect, to synthesize their proteins. They are intracellular parasites that depend on the infected cell in order to realize their replication. Viruses are generally very simple shapes, composed of protein and nucleic acid , although their structure and composition are highly variable. Are classified, according to leading into the nucleic acid: DNA viruses and RNA viruses, and according to their form and structure in different families.
(CS): single chain. (Cd): double chain
From the immunological point of view, are interested to know the viral replication cycle, to predict the opportunities that have different immune mechanisms to interact with the viral particle, the infected cells, or both. Normally, the viral replication cycle begins by ' union of the virus (virus free) to the host cell through specific receptors (absorption) (1), these receptors determine the tropism and specificity of infection (they can not infect any cell or any species, have their specific tropism), a Once in the cell, the virus removes its cap by releasing the nucleic acid (elimination capsule) (2), to begin the process of viral replication.
At this stage, the synthesis of cellular proteins is inhibited and will be processed only the genetic information of the virus. The mechanisms that act at this stage depends on the type of virus nucleic acid (DNA or RNA). In the case of virus DNA replication occurs (3), forming a new viral DNA. The new viral DNA, through transcription, changes in viral RNA (blue), which through translation, will implement the various viral proteins and then go viral assembly (4). In the case of RNA viruses, transcription is not necessary, you can go directly from the production of new RNA viral proteins. The mechanism of RNA replication, is different for retroviruses, such as from viral RNA by a reverse transcriptase, form viral DNA (joins the cellular genome) starting from which the different phases of replication, etc..
In most viral infections, the immune system has the opportunity to counteract the virus particle, at certain times of infection (prior to entering the cell or leave it, after replication), and the infected cells (during production of viral proteins or the assembly), since they appear in the infection of the membrane antigens that trigger the immune response. In some cases, such as pig endogenous retroviruses (have been described three types: A, B, B1 and C), or in the herpes virus (Aujeszky's disease), the infection can circulate for long periods of time without giving you a virus particle, or that infected cells express membrane antigens. In these moments, the mechanisms of the system immunity are ineffective, because the enemy does not offer any kind of signal, but at a given time (not familiar with all the circumstances) infection and reactive free new infectious virions.
From the point of view of immunology, viral infections can be fought, - once through chemical-physical barriers, fighting against the virus particle (virion), against the infected cells or against both, through different mechanisms of response to natural and acquired.
Response to natural virus
The mechanisms of the most active natural response against viral infections are mediated by 'interferon and activation of NK cells. These mechanisms are directed to the infected cells.
The interferon is a cytokine of which there are three types, called a, b and g. The first two are produced primarily by monocytes-macrophages and to a lesser extent by fibroblasts, and interferon-g is produced by lymphocytes and CD4 CD 8 and NK cells. Interferon has a large capacity and induce different antiviral mechanisms such as: transient resistance of the cells, the induction of different molecules with antiviral activity , activation of genes that express antiviral proteins and an increase of expression of SLA I and SLA II .
NK cells are activated against cells naturally infected by the virus. The mechanism of activation appears to be related to alterations in the expression of SLA in infected cells. The response of NK cells are infected, not based on a reaction antigen (TcR do not have to NK). This cytotoxic mechanism is very effective in viral infections.
Finally, the alternative pathway of complement activates also the lysis of virus particles with great efficiency.
Acquired response against the virus.
The acquired immunity reacts against viral infections, both viral particle, is against the infected cell. Against the viral particle, the most important immunological mechanism, the antibodies, while against the infected cells are the cytotoxic mechanisms mediated by cell ( CD 8 + ) or by antibodies and cells ( ADCC ) or antibody and complement (classical pathway).
Against the viral particle .
The capsid of the virus particle is composed of proteins, which are antigenic and induce a large amount of antibodies that may have different actions against viruses:
Neutralize the infection ( IgG, IgM and IgA ), preventing the virus to enter cells.
Clumping virus (IgM), reducing the number of available units.
Activation of phagocytosis when forming the antigenantibody complex and stimulate the Fc-Receptor of macrophages.
Against the infected cell.
The infected cells can articulate on their membrane antigens, long before you produce the viral assembly. Their destruction is an excellent mechanism to prevent the formation of other viruses acquired .. The answer is in contrast to cells infected with both antibodies (ADCC system, activation of the complement classical pathway, activation of phagocytosis) and cellular cytotoxicity of lymphocytes through the CD 8 +, which is one of the most effective mechanisms against viral infections.