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30.2.3 How viruses deceive immune system?

Viruses have adopted numerous strategies for escaping the immune system.

•  Viruses can change their surface antigens to avoid immune response. Generally surface glycoproteins containing T-cell epitopes undergo changes by point mutation or reassortment of genes especially in RNA viruses.

•  Some viruses escape the immune surveillance by inhibiting the antigen presentation process and by inactivating the immunocompetent cells.

•  Suppression of immunosuppressive molecules is also one of the strategies adopted by viruses.

30.3 Immunity to Parasites

Parasitic infections are mostly the infections caused by protozoa, ectoparasites and helminths. The parasitic infections are mostly chronic because of weak innate immunity. Besides weak immunity, parasites have a knack of evading host immune response very easily.

30.3.1 Innate immunity to parasites

Phagocytosis is the main innate immune response to parasitic infections but many parasites are able to escape the immune system. E.g. some helminths have thick teguments that enable them to evade the cytocidal mechanism of neutrophils and macrophages. Very few parasites have the potential to activate alternate pathway of complement system but the parasites that recoup from infected patients acquire resistance to complement mediated lysis.

30.3.2 Adaptive immunity to parasites

Parasites exhibit diverse adaptive immune response. Cell mediated immunity is the principal defense mechanism against parasitic infections. Stimulation of macrophages by Th1 cell derived cytokines is especially directed by cell mediated immunity to neutralize the antigens. Helminths are removed by IgE antibody and eosinophil-mediated killing as well as other leukocytes.