5 Summary

This week you learnt about the elements of the adaptive immune system that combat a virus infection. Cytotoxic T lymphocytes and NK cells act in a complementary fashion to detect virus-infected cells of the body. If they recognise an infected cell they signal to it to induce apoptosis, - programmed cell death.

B cells can recognise viral antigens and, with help from TH2 cells, they divide and differentiate into plasma cells, which secrete antibody. Individual B cells switch from production of IgM to IgA or IgG. The different antibody classes have different functions. IgM is produced first in an immune response; IgG is the major antibody in the secondary immune response and IgA can be secreted across epithelial cells to protect mucous membranes.

Antibodies protect against virus infection in a number of ways. Neutralising antibodies prevent virus from attaching to target cells. IgG and IgM antibodies can activate complement to damage enveloped virus or virus-infected cells. IgG can promote phagocytosis of virus by macrophages and allows NK cells to recognise infected cells. If they are internalised by an infected cell, IgG antibodies can interfere with virus replication and assembly, as well as promoting presentation of the viral antigens on MHC class 1 molecules, to cytotoxic T cells.

Clearly, antibodies are very important in protection against virus infection. Indeed, the vaccines against COVID-19 were designed to induce high levels of neutralising antibodies against the SARS CoV2 spike protein. But how does one measure antibodies? That is what you will learn about next week, using a technique called ELISA, in a virtual laboratory.

You should now go to Week 3.