T lymphocytes are white blood cells (leukocytes) that are part of the immune system to help fight disease. There are two main types of immune responses: the innate (non-specific) immune response and the adaptive (specific) immune response, with T lymphocytes being involved in the latter(also known as T cell immunity). They are first made in the bone marrow and mature in the thymus, a small organ found between the lungs. There are four types of T lymphocytes, each of which has its unique function, which we will explore in the following sections.
T lymphocytes are also known as T cells!
The role of T lymphocytes in the immune response
As previously mentioned, there are four types of T lymphocytes that each perform their own task to fight off pathogens in the body. These T lymphocytes are:
- Helper T lymphocytes
- Cytotoxic T lymphocytes
- Regulatory T lymphocytes
- Memory T lymphocytes
Helper T lymphocytes
Helper T lymphocytes work together with B lymphocytes, another type of white blood cell involved in the adaptive immune response. Helper T lymphocytes secrete cytokines that stimulate the activation of B lymphocytes so that they'll produce antibodies against the pathogen.
Cytokines are proteins secreted by immune cells to signal and regulate the immune response.
Antibody: Proteins secreted by B lymphocytes that neutralise, opsonise and agglutinate antigens and pathogens, helping to clear the infection.
Cytotoxic T lymphocytes
Cytotoxic T lymphocytes, also known as killer T lymphocytes, directly destroy the invading pathogen by secreting the perforin proteins and granzymes. Perforin creates pores in pathogens' cell membranes, allowing granzymes to enter them and induce apoptosis, which enables the destruction of the pathogen itself and its infected cells!
Apoptosis: programmed cell death, which does not induce an inflammatory response.
Fig. 1 - The mode of action of cytotoxic T lymphocytes against virally infected cells
Response to virally infected cells
The release of perforin and granzymes allows the clearance of virally infected cells. In addition, cytotoxic T lymphocytes also secrete another cytokine called IFN-γ (γ is the Greek letter gamma). This cytokine inhibits viral replication and induces macrophage activation, both of which help to clear viral infections.
Cytotoxic T lymphocytes are also called CD8+ T cells as they express a glycoprotein called CD8 on their plasma membrane. This glycoprotein enables the cell to bind to antigens on the surface of infected cells.
Regulatory T lymphocytes
Regulatory T lymphocytes work by suppressing the immune response, preventing helper and cytotoxic T lymphocytes from continually being activated, and limiting their reaction once the pathogen has been cleared.
If the immune response isn't stopped or regulated correctly, it can cause different diseases. When the immune system attacks its own body, this is called an autoimmune disease. When the immune system recognises antigens that aren't dangerous for the body, the following immune reaction causes allergies.
Memory T lymphocytes
Memory T cells form part of the secondary immune response, which involves the rapid response to antigens after initial exposure. They are long-lived cells as they reside in the bloodstream for an extended period. When they detect the presence of the antigen they are complementary to, they differentiate rapidly into helper and cytotoxic T lymphocytes to clear the pathogen.
The primary immune response describes the initial exposure to a new antigen. For example, when you are first infected with the chickenpox virus, the primary immune response involves the activation of T lymphocytes and B lymphocytes. This response is generally very slow, but during this time, memory T lymphocytes are formed (as well as memory B lymphocytes).
If you are exposed to the chickenpox virus a second time, your secondary immune response kicks in, and this is a more rapid response than the primary immune response. Your memory T lymphocytes are activated and differentiate into different effector cells very quickly!
T lymphocytes in cell-mediated immunity
Cell-mediated immunity describes the immune response that doesn't involve antibodies. The body fights back just with mature T cells, macrophages and cytokines. T cells can recognise other cells that have been infected and modified by a pathogen. This is because infected cells will present the pathogen's antigens on the surface of their plasma membrane. These are known as antigen-presenting cells (APCs), and T lymphocytes detect them.
Infected cells present pathogenic antigens on their major histocompatibility complex (MHC), which is found on the surface of their plasma membrane. There are two classes of MHC's: MHC-I and MHC-II.
Outlined below are the stages involved in cell-mediated immunity involving T lymphocytes.
- A pathogen (for example, a virus) invades the body.
- Phagocytes engulf the pathogen and present the antigen on the surface of their plasma membrane.
- Receptors on the surface of helper T lymphocytes detect and fit the antigen.
- The helper T lymphocytes bind to the antigen and become activated.
- The helper T lymphocytes secrete cytokines to differentiate into cytotoxic T lymphocytes, differentiate into memory T lymphocytes, differentiate into regulatory T lymphocytes and stimulate phagocytosis. They can also activate B lymphocytes, but they are a part of humoral immunity!
There are three primary phagocytic cells: macrophages, neutrophils and dendritic cells.
Fig. 2 - Cell-mediated immunity and the outcomes
Differences between cell-mediated immunity and humoral immunity
The adaptive immune response involves cell-mediated immunity and humoral immunity. We suggest you read our article Humoral Immunity to understand its importance but to summarise, humoral immunity involves the B lymphocytes response to infection. The main differences between these immune responses include:
- Cell-mediated immunity involves only T lymphocytes and macrophages. Humoral immunity involves B lymphocytes.
- Cell-mediated immunity responds to cells that have been modified by engulfed pathogens. Humoral immunity responds to extracellular pathogens.
- Cell-mediated immunity does not involve the production of antigen-specific antibodies, while humoral immunity does.
- Cell-mediated immunity destroys pathogens by inducing apoptosis. Humoral immunity destroys pathogens by secreting antibodies that neutralise, opsonise and agglutinate antigens.
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