What are the different types of T-cells?

There are several different types of T-cells. They are largely grouped into two categories: 

Cytotoxic T-cells 

Cytotoxic T-cells play a critical role in the immune system's defense against infections and cancer. 

The surface of a cytotoxic T-cell is equipped with a specific T-cell receptor (TCR), which allows it to recognize foreign antigens displayed on the surface of infected or abnormal cells. On encountering an antigen that matches its T-cell receptor, the cytotoxic T-cell becomes activated and multiplies rapidly forming an army of effector T-cells, each with the same specificity for the recognized antigen. The effector cytotoxic T-cells release specialized proteins that work via various mechanisms to destroy the infected cells. The regulation and function of cytotoxic T-cells are tightly controlled to avoid attacking the body's healthy cells.

Cytotoxic T-cells are also called CD8+ cells because they must be accompanied by a glycoprotein called CD8 in order to the bind to the constant portion of the class 1 major histocompatibility complex (MHC). This is important because T-cells can only recognize an intruder when it’s attached to MHC.

Helper T-cells

Helper T-cells play a more supportive role in the immune system.  Although they don’t kill infected cells directly, they help cytotoxic T-cells and B cells by sending signals to other cells and coordinating effective immune responses against invading pathogens such as bacteria, viruses, and fungi.

Activated helper T-cells primarily fulfill two essential functions. They provide crucial help to B-lymphocytes in the production of antibodies. They play a major role in activating cytotoxic T-cells and enhancing their ability to recognize and eliminate infected or cancerous cells more effectively.

Helper T-cells are also called CD4+ cells because of the presence of a CD4 receptor on their membrane surface.