What is a T cell?

• T cell, also called T lymphocyte, is a type of leukocyte (white blood cell) that is an essential part of the immune system.
• T cells are one of two primary types of lymphocytes—B cells being the second type—that determine the specificity of immune response to antigens (foreign substances) in the body.

Origin and differentiation

• T cells originate in the bone marrow and mature in the thymus. 
• In the thymus, T cells multiply and differentiate into helper, regulatory, or cytotoxic T cells or become memory T cells. 
• They are then sent to peripheral tissues or circulate in the blood or lymphatic system.

Functions of T cells

• Once stimulated by the appropriate antigen, helper T cells secrete chemical messengers called cytokines, which stimulate the differentiation of B cells into plasma cells (antibody-producing cells). 
• Regulatory T cells act to control immune reactions, hence their name. 
• Cytotoxic T cells, which are activated by various cytokines, bind to and kill infected cells and cancer cells.
• Because the body contains millions of T and B cells, many of which carry unique receptors, it can respond to virtually any antigen.

Why in News?

• Natural exposure or infection with the novel coronavirus may “prevent recurrent episodes of severe COVID-19”, says a paper published in Cell by a Swedish institute. 
• This is because, once infected with SARS-CoV-2, the immune system elicits “robust, broad and highly functional memory T cell responses”.

More in the News

The study found SARS-CoV-2-specific T cells even in family members who have been exposed to the virus but have tested negative on antibody blood tests. 

SARS-CoV-2-specific T cells were also seen in convalescent (recovered) individuals with a history of asymptomatic infection and mild COVID-19 disease.

Significance of the study

The researchers were able to detect similar memory T cell responses directed against the internal and surface proteins (membrane and/or spike) of the virus in some people in whom SARS-CoV-2-specific antibodies could not be detected.

Studies about development of natural immunity against Covid-19 have largely been limited to prevalence of antibodies.

Reference:

1. https://www.thehindu.com/sci-tech/science/sars-cov-2-specific-t-cell-immunity-prevents-recurrent-severe-covid-19-disease/article32363678.ece