An individual is exposed to many infectious agents, however, in most situations does not develop a disease thanks to factors such as physical and chemical host barriers. If the virus is able to overcome these first lines of defense, a type of highly specialized and specific protection will be activated. This defense will achieve, in most situations, the infection control and subsequent eradication of the disease.
Innate immune defense provides the first line against viral infections. The innate immune system is composed mainly of physical barriers, chemical barriers, innate immune cells, and soluble mediators such as the complement system, innate antibodies, and associated cytokines. The cells mediating innate immunity do not respond to specific viral antigens as do their counterparts in the adaptive immune response. Several distinct activities mediate innate immune defense against viruses, including:
The killing of virus-infected cells by cytotoxic T lymphocytes (CD8+ T cells) is achieved by the release of cytotoxic granules at the interface between the receptor-bound T cell and a virus-infected cell including perforin, granzymes, and granulysin. Cytotoxic CD8 T cells can also release cytokines that act either locally or at a distance to impact virus infection, such as IFN-γ, which can block virus replication and even eliminate the virus from infected cells without inducing cell death.
A key function of effector CD4 T cells is to provide help to CD8 T cells, an essential step in the activation of cytotoxic T lymphocytes in the majority of viral infections. In addition to the helper function, CD4+ T cells also display cytolytic activity.
Fig.1 Induction of humoral and cellular immunity in the viral infection. (Van de Sandt, 2012)
Each virus can cause different cytopathic effects in the host cell, which may lead to several symptoms and disease. There are some strategies against viral infections:
Fig.2 The viral life cycle and drug interaction sites. (Slagman, 2021)
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