A recent study has shown that people unexposed to and not infected with novel coronavirus may still exhibit T cell responses specific to this virus. It is thought that Coronavirus specific T cell responses seen in healthy people might arise from memory T cells derived from exposure to ‘common cold’ coronaviruses.

Try this question from CSP 2010:

Q. Widespread resistance of malaria parasite to drugs like chloroquine has prompted attempts to develop a malaria vaccine to combat malaria. Why is it difficult to develop an effective malaria vaccine?

(a) Malaria is caused by several species of Plasmodium

(b) Man does not develop immunity to malaria during natural infection

(c) Vaccines can be developed only against bacteria

(d) Man is only an intermediate host and not the definitive host

What are T Cells?

• T Cells also called T lymphocyte, 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 the immune response to antigens (foreign substances) in the body.
• T cells originate in the bone marrow and mature in the thymus.

Why are they called memory cells?

• 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.
• 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).

How do they control immunity?

• 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.

Vaccination outcomes on T cells

• There is a possibility that pre-existing T cell memory might influence vaccination outcomes.
• Pre-existing immunity could help elicit better immune responses against novel coronavirus, and these responses can manifest faster.
• Meanwhile, pre-existing immunity could be mistaken as an enhanced efficacy of the vaccine in eliciting immune responses.
• This could be particularly confusing in Phase-1 trials where the vaccine is tested on a small group of healthy participants.

Its drawbacks

• The pre-existing immunity can reduce the immune responses that the vaccine causes through a mechanism called the “original antigenic sin”.
• It can also lead to antibody-mediated disease enhancement, where antibodies present at sub-neutralizing concentrations can actually augment virus infection and cause more severe disease.
• This was seen in the case of chikungunya and dengue.

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