T cells/T lymphocytes are white blood cells produced in the thymus gland. They play an important role in adaptive immunity. There are several subtypes of T cells. These include: T helper cells, T regulatory cells, T memory cells and cytotoxic T cells. These cells have distinct functions and they work together in a complex network involving other immune cells to combat disease. This article will cover the role of the T memory cell and how they help to protect against infection in the long term.
Initial Exposure
Innate T cell Immune Response
When an antigen first enters the body it encounters cells of the innate immune system, e.g. macrophages and dendritic cells. These cells capture and present the antigen (antigen presenting cells/APCs) and release a variety of cytokines and inflammatory mediators to recruit other immune cells and to stimulate the innate immune response.
The antigen presenting cells then drain into local lymph nodes where they encounter naïve T helper cells and B cells. These initiate the more specialised, adaptive immune response. The antigen exposure causes the naïve T helper cells to differentiate into memory helper T cells. These then proliferate and specialise into Th1 or Th2 roles – leading to activation of cytotoxic T cells and B cell differentiation.
Adaptive T cell Immune Response
The cytotoxic T cells help with pathogen removal. They recognise and bind to the antigens expressed by pathogens via MHC I molecules. Upon binding to the antigen/pathogen the cytotoxic T cell releases a variety of mediators to destroy the pathogen.
The T helper cells release a variety of cytokines which activate cytotoxic T cells and macrophages and induce B cell differentiation into plasma cells to produce antigen specific antibodies. These antibodies help to fight the infection through binding to the antigen. Once bound, the antibody prevents the antigen from binding to other targets and also facilitates antigen recognition and removal.
Secondary Exposure
T Memory Cells
These cells serve to ‘remember’ the specific antigen involved in this encounter, so that should this antigen enter the body again the T helper cells would be able to activate B cells much faster. Subsequently, antigen-specific antibodies are produced.
The T helper cells would also stimulate faster expansion of cytotoxic T cells to hasten pathogen clearance from the body. This would lead to a far quicker immune response and faster infection clearance.
Clinical Relevance – Vaccinations
The principle of T cell memory is exploited during vaccinations. A vaccine contains weakened or dead forms of the pathogen and is designed to mimic the initial exposure response. Upon entry, the dead or weakened pathogen stimulates the innate immune response as the antigens are captured and presented by APCs. Consequently, this leads to the differentiation of naïve T cells and expansion of the T cell and B cell population.
Although there is no clinical infection to fight in this scenario, the vaccine enables the production of T memory cells so that should a live form of this pathogen enter the body the immune system is already prepared to fight it.
Fig 1 – Graph showing the immune response in the body following vaccination and reinfection – the response is much faster on reinfection due to B and T cell memory cells in the immune system