The Immune System and the Role of T Cells

By Dr Clément Monteil

When the human body encounters an external infection such as a virus, bacteria or parasite, it activates a complex and coordinated defence mechanism to identify, target and eliminate the invading pathogen. This response is managed by the immune system, an elaborate network consisting of cells, organs, and molecules, designed to protect against infections and diseases.

The immune system functions through two primary levels: innate and adaptive immunity. Innate immunity acts as the first line of defence, providing a rapid but non-specific response to the pathogens. It includes physical barriers like the skin and mucous membranes, alongside immune cells such as macrophages and neutrophils, which consume and neutralize infectious agents. It also involves proteins designed to disrupt them, ensuring an immediate protective response. Adaptive immunity, in comparison, delivers a precise and targeted response to specific infectious agents. This system has a remarkable ability to remember past infections, allowing for a quicker and more effective response if the same pathogen is encountered again. This aspect of the immune system demonstrates an impressive ability to adapt and respond more effectively over time.

Lymphocytes are a type of white blood cell that are central to the adaptive immune system, specifically the B-cells and T-cells. B-cells are responsible for producing antibodies, which bind to pathogens and mark them for destruction.

T-cells play a more direct role in killing infected cells or helping other immune cells to perform their functions more effectively. They are differentiated from the other lymphocytes by their development in the thymus and their critical roles in adaptive immunity. They circulate throughout the body, moving from the bloodstream into various tissues, especially the lymph nodes and spleen, where they are primed to respond to pathogens. This means that these cells have been activated and prepared to recognize and fight against specific infectious agents. They are essential for the body to mount an effective defence against many types of infections and diseases.

T-cells are classified into several types based on their functions and the presence of specific surface molecules, known as cluster of differentiation (CD) markers. The main types include:

• CD4+ T-cells, also known as helper T-cells, assist other cells in the immune system; they produce a variety of cytokines to orchestrate the immune response, including supporting B cell antibody production, activating macrophages, and helping activate cytotoxic T cells.
• CD8+ T-cells, or cytotoxic T-cells, are capable of directly killing infected or malignant cells by producing cytotoxic molecules (such as perforin and granzymes), and secreting effector cytokines.
• Regulatory T-cells (Tregs) help control the immune response to avoid attacking the body’s own tissues or responding inappropriately to harmless substances, such as food.
• Memory T-cells can remember past infections. Long after a pathogen has been eliminated, memory T-cells can undergo rapid proliferation and differentiate into effector T cells to mobilise a fast, specific immune response upon reinfection.

Understanding the importance of T-cells has practical applications in medicine and biotechnology. Testing T-cells provides invaluable information about the immune status and potential vulnerabilities of an individual, informing both diagnosis and treatment strategies for a wide range of diseases. Various markers, including CD markers and cytokines (proteins secreted by cells that influence cell behaviour), are used to identify and characterize T-cells and their function. Different testing methods are employed to assess these markers and the overall function of T-cells. Flow cytometry is a technique that provides a detailed analysis of cells, detecting different surface and intracellular markers using fluorescently labelled antibodies. Enzyme-linked immunosorbent assay (ELISA) is used to quantify the presence of cytokines and other proteins in plasma samples, providing insights into the immune response. Finally, ELISpot and FluoroSpot assays offer a way to detect and quantify the production of cytokines by individual T-cells, giving a view of the cellular-level adaptive immune response to an infection.