T-cell immunity is a key component of the adaptive immune response against pathogens and virus-infected cells.
The two main types of effector T cells are helper T cells and cytotoxic T cells. Helper T cells act as immune coordinators and secrete cytokines to activate other immune cells (such as macrophages, B cells, and cytotoxic T cells). Cytotoxic T cells release cytotoxic granules to directly destroy infected cells and pathogens.
Memory T cells are formed from cytotoxic T cells following the adaptive immune response and give the pathogen long-term immunity.
What are T cells?
T cells are one of the two main types of Lymphocytes (white blood cells) of the adaptive immune system. Like B cells, they come from the bone marrow, but unlike B cells, they mature in the bone marrow Thymus. There are two main types of effector T cells and these are Helper T cells and cytotoxic T cells. In the event of an infection, T cells work with each other and with other immune components to trigger a targeted reaction against the pathogen.
Activation of T cells
T cells come from stem cells in the bone marrow and mature in the Thymus. Until they meet they are called their specific antigen naive T cells.
T cells are specific for only one type of antigen, but are not activated directly by the antigen. Antigen presenting cells (APCs) such as macrophages, dendritic cells, and T cells mediate activation of T cells by engulfing invading pathogens. Once engulfed, the pathogen will be digested into protein fragments, and some of these will appear as part of yours on the surface of the APC Major Histocompatibility Complex (MPC).
The APCs then make their way to the thymus, where they present the antigens on their surface to naive T cells. When a T cell recognizes the antigen, it binds to the APC via the MHC receptor.
Types of T cells
The two main types of T cells are Helper T cells and cytotoxic T cells (AKA ‘killer’ T cells). Both types of lymphocytes originate from the bone marrow and mature in the thymus, but express different types of co-receptors on their surface.
All T cells express a T cell receptor (TCR) and either a CD4 or a CD8 co-receptor. Helper T cells express CD4 receptors and cytotoxic T cells express CD8 receptors. For this reason, helper T cells and cytotoxic T cells can also be referred to as CD4 + and CD8 + T cells, respectively.
The type of CD receptor that a T cell expresses on its surface determines which type of APC it can bind to. This is because CD4 + receptors can only bind to MHC class II complexes (MHCII complexes) and CD8 + receptors can only bind to MCHI. This process activates naive T cells to multiply and become Effector cells; either helper (CD4 +) T cells or cytotoxic (CD8 +) T cells.
The third type of T-cell, memory T-cell, is formed after the adaptive immune response. The cytotoxic T-cell counts peak about a week after the initial infection, before they are in the Contraction phase. Remaining cytotoxic T cells remain in the immune system as memory T cells and give the pathogen long-term immunity.
Functions of T cells
Helper T cells
Helper T cells play a central role in the adaptive immune response, but do not attack pathogens directly. Instead, they secrete cytokines that activate and coordinate other immune cells (such as macrophages, B cells, and cytotoxic T cells) to launch an attack against foreign invaders.
Cytotoxic T cells
Cytotoxic T cells are also known as “killer” T cells because they directly attack and destroy pathogens and virus-infected host cells. When they find an abnormal cell, these lymphocytes release cytotoxic granules that contain Perforin and Granzyme, two proteins that work together to destroy the infected cell. Perforin causes pores to develop in the target cell membrane and granzyme to enter the cell through these holes. Once inside, the granzymes trigger apoptosis and kill the host cell and all viruses living in it.
Memory T cells
Memory T cells are cytotoxic T cells that remain in the body after an infection is cleared. Once activated, cytotoxic T cells multiply quickly and migrate to the infected part of the body. The number of cytotoxic T cells typically peaks about 7 days after the initial infection, after which they are in an as Contraction phase.
All cytotoxic T cells that survive the contraction phase remain in the body as memory T cells over the long term. They “remember” the pathogen and react quickly in the event of renewed infection.