T-cell immunity, also known as cellular immunity, is a branch of the immune system that involves the activation and function of T cells in response to pathogens, such as viruses and bacteria, as well as abnormal cells like cancer cells. T cells play a crucial role in recognizing and destroying these foreign or infected cells, thereby protecting the body from infection and disease. There are two main types of T cells involved in cellular immunity: cytotoxic T cells, which directly kill infected or abnormal cells, and helper T cells, which assist in the activation of other immune cells. T-cell immunity is essential for the body's ability to mount an effective response against pathogens and maintain overall health. Research in the field of T-cell immunity aims to understand the mechanisms by which T cells recognize and respond to specific antigens, as well as how these responses can be modulated or enhanced for therapeutic purposes, such as in the development of vaccines or immunotherapies for cancer. Additionally, researchers explore how T-cell immunity can be dysregulated in autoimmune diseases or suppressed in conditions like HIV infection.