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 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 serves as the first line of defence, providing a rapid but non-specific response to pathogens. It includes physical barriers like the skin and mucous membranes, alongside immune cells such as macrophages and neutrophils that engulf and neutralise infectious agents. Additionally, innate immunity involves proteins that disrupt these pathogens, ensuring an immediate protective response.
Adaptive immunity, in comparison, provides 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 characteristic 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 produce antibodies that bind to pathogens and mark them for destruction. In contrast, T cells have a more direct role in killing infected cells or helping other immune cells perform their functions more effectively.
T cells are differentiated from the other lymphocytes by their development in the thymus and their essential 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. T cell priming and activation are critical steps in the immune response, enabling T cells to recognise and combat infections effectively.
T cell priming involves three key signals. First, the T cell receptor binds to a peptide-MHC complex on the surface of an antigen-presenting cell, providing specificity for the immune response. The interaction between co-stimulatory molecules on the antigen-presenting cell and receptors on T cells ensures full activation and prevents anergy (a state of immune unresponsiveness). Additionally, cytokines (proteins secreted by cells that influence cell behaviour) are released by antigen-presenting or surrounding cells to guide the differentiation of T cells into specific effector subsets.
Different T cell subsets can be categorised 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, 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.
