Helper T cells are one of the two major types of T lymphocytes in the immune system and play a key role in the coordination of the immune response. They are central to adaptive immunity and are involved in almost all adaptive immune responses, as they secrete cytokines for the proliferation and stimulation of other immune cells.
What is a T Cell?
T cells and B cells are the two major types of lymphocytes (white blood cells) in the immune system. T cells can be further divided into two main types of effector cells; these are helper T cells and cytotoxic (AKA ‘killer’) T cells.
Both T cells and B cells originate from stem cells in the bone marrow, but they mature in different regions of the body. B cells mature in the bone marrow, whereas T cells mature in the thymus. Although both types of lymphocytes play essential roles in adaptive immunity, their functions are very different.
Whereas B cells secrete antibodies, helper T cells secrete cytokines that stimulate other immune cells (such as B cells, macrophages, and cytotoxic T cells) to attack infectious agents. Cytotoxic T cells have a different function and produce toxins that kill infected cells and pathogens directly.
What is a Helper T Cell?
Helper T cells are one of the two major types of T lymphocytes in the human immune system. Both helper T cells and cytotoxic T cells develop from stem cells in the bone marrow and both mature in the thymus. The main differentiation between the two is the receptor molecules they display on their surfaces.
As they mature in the thymus, T cells develop T Cell Receptors (TCRs) on their surface alongside co-receptors called CD4 and CD8 receptors. All T cells express TCRs, but only helper T cells express CD4, and only cytotoxic T cells express CD8. For this reason, helper T cells are also known as CD4+ T cells.
Helper T Cell Activation
A newly mature T cell is called a naïve T cell until it encounters an antigen and is activated. The TCR on the surface of the T cell is specific for one particular type of antigen; however, it cannot bind to antigens directly. They can only recognize antigens that are bound to Major Histocompatibility Complex Class 1 (MHCI) and Class 2 (MCHII) receptor molecules.
MHC receptor molecules are expressed by antigen-presenting cells (APCs), such as B cells, macrophages, and dendritic cells. When an APC encounters a pathogen, it will take peptide fragments from the pathogen and display these on its surface as part of its MHC complex.
When a naïve T cell recognizes an antigen on the surface of an APC, both the TCR and CD receptors bind to the MHC complex. CD4 receptors bind only to MHCII complexes, and CD8 receptors bind only to MHCI complexes. During this process, both CD4+ helper T cells and CD8+ cytotoxic T cells are activated to become effector cells.
Helper T Cell Function
Helper T cells play a central role in the functioning of the adaptive immune system and are required for almost all adaptive immune responses. They don’t secrete toxins or antibodies, and they don’t directly attack pathogens. Instead, they ‘help’ by secreting cytokines that activate various other immune cells and coordinate the immune response against infectious agents.
Helper T Cells Secrete Cytokines
Once activated, helper T cells start to secrete proteins called cytokines, and these activate virtually all the other cells of the immune system. Cytokines stimulate the proliferation and differentiation of other immune cells such as T cells, macrophages, B cells, and leukocytes. In doing so, helper T cells mediate the immune response against infection.
Helper T Cells Activate Phagocytes and Cytotoxic T Cells
The cytokines produced by helper T cells also stimulate cytotoxic T cells and phagocytes (such as macrophages). Phagocytes engulf and destroy infected cells, while cytotoxic T cells release toxins to kill their targets. By activating these cells, helper T cells play a vital role in mediating the immune system’s response against infectious agents.
Helper T Cells Stimulate B Cells
Helper T cells stimulate B cells to proliferate and differentiate into memory B cells and plasma B cells, which secrete antibodies against invading pathogens. The antibodies circulate in the blood until they encounter their corresponding antigen and bind to form an antigen-antibody complex. In some cases, this ‘neutralizes’ the pathogen by blocking surface sites necessary for infectivity. Other times, the antibodies produced by B cells will tag the pathogen for destruction by other immune cells (such as plasma cells).
Helper T Cells vs. Cytotoxic T Cells
Helper T cells and cytotoxic T cells are the two major types of T cells in the human immune system. Both develop from stem cells in the bone marrow, and both mature in the thymus, but their roles as effector cells are very different.
Helper T Cells
Cytotoxic T Cells
|Develop from||Stem cells in the bone marrow||Stem cells in the bone marrow|
|Mature in||The thymus||The thymus|
|Function||Secretes cytokines to co-ordinate immune response||Secretes toxins to directly kill infected cells and pathogens|