T Lymphocytes

Introduction

T lymphocytes (T cells) are a type of white blood cell that play a crucial role in the adaptive immune response. They are responsible for recognizing and eliminating infected cells, cancer cells, and foreign substances in the body. This article provides an overview of T lymphocytes, their types, functions, and their significance in clinical settings.

Types of T Lymphocytes

1. Helper T Cells (CD4+ T cells): These T cells express the CD4 receptor on their surface and play a critical role in coordinating immune responses. They recognize antigen-presenting cells (APCs) and secrete cytokines to activate other immune cells, such as B cells and cytotoxic T cells.

2. Cytotoxic T Cells (CD8+ T cells): These T cells express the CD8 receptor on their surface and are responsible for directly killing infected cells or tumor cells. They recognize antigen fragments presented on the surface of infected cells in complex with major histocompatibility complex class I (MHC-I) molecules.

3. Regulatory T Cells (Tregs): Tregs play a crucial role in maintaining immune tolerance and preventing autoimmune reactions. They suppress immune responses to self-antigens and help control excessive immune reactions.

T Cell Receptors

T lymphocytes possess unique T cell receptors (TCRs) on their surface, which enable them to recognize specific antigens. TCRs are composed of two protein chains: alpha (α) and beta (β) chains. The diversity of TCRs allows recognition of a wide range of antigens.

T Cell Activation

1. Antigen Recognition: T cells recognize antigens presented by APCs. Helper T cells recognize antigens presented on MHC-II molecules, while cytotoxic T cells recognize antigens presented on MHC-I molecules.

2. Co-stimulation: T cell activation requires co-stimulatory signals, such as interaction between CD28 on T cells and B7 on APCs. Without co-stimulation, T cells may become tolerant or undergo apoptosis.

3. Clonal Expansion: Upon activation, T cells undergo clonal expansion, resulting in the proliferation of antigen-specific T cells. This amplifies the immune response and ensures sufficient T cell numbers to combat the infection.

Clinical Significance

1. HIV/AIDS: Human Immunodeficiency Virus (HIV) specifically targets and infects CD4+ T cells, leading to a progressive decline in immune function. This results in acquired immunodeficiency syndrome (AIDS), making patients susceptible to opportunistic infections and malignancies.

2. Transplant Rejection: T cells play a significant role in transplant rejection as they can recognize differences between self and non-self antigens. Effector T cells can attack transplanted organs, leading to graft rejection.

3. Autoimmune Diseases: Dysregulation of T cell responses can lead to autoimmune diseases, where T cells mistakenly attack self-antigens. Examples include rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus.

Conclusion

T lymphocytes are key players in the adaptive immune response. Their ability to recognize specific antigens, coordinate immune responses, and eliminate infected or abnormal cells is essential for maintaining a healthy immune system. Understanding T lymphocytes and their functions is crucial for medical professionals in diagnosing and treating various immune-related disorders.