T-cell subtypes are specialized immune cells that play crucial roles in the body's defense against infections and diseases. The main types include CD4+ T cells, which help coordinate the immune response, and CD8+ T cells, which directly kill infected or cancerous cells. Each subtype has unique functions and markers that help identify them. Another important subtype is regulatory T cells (Tregs), which help maintain immune system balance by preventing excessive responses that could harm the body. Additionally, memory T cells are formed after an infection, allowing for a quicker response if the same pathogen is encountered again in the future.