Major Effector T Cells

Major Effector T Cells

Activated T cells, similar to B cells, may become effector cells or memory cells. The three major types of effector T cells are helper, cytotoxic, and regulatory T cells.

Helper T Cells

Helper T cells, abbreviated asT_Hcells, are central toadaptive immunity. They mobilize both its humoral and cellular components. When activated by APC activity, T_H cells help to activate both B and T cells. They also “encourage” B and T cells to proliferate. Without the aid of these “director” T_H cells, there will be absolutely no adaptive immune response. Their cytokines provide chemical assistance require to recruit other immune cells. To understand just how important the T_H cells are in immunity, consider what happens when they are destroyed, such as in the con-dition known as AIDS.

There is direct interaction between T_H cells and B cells that display antigen fragments bound to class II MHC receptors. When a T_H cell binds to a B cell, the T_Hcell releases cytokines that encourage the B cells todivide more quickly. The T_H cell then signals antibody­formation to start. B cells continually divide as long as they are stimulated by T_H cells. Therefore, T_H cells help to release the ability of B cells to protect the immune system.

Certain B cells are activated only when they bind to T cell-independent antigens. However, responses to these antigens are usually weak and of short dura-tion. Most antigens are T cell-dependent antigens. They require T cell assistance in order to activate the B cells that they bind to. Similar to B cells, CD8 cells usually need assistance from T_H cells in order to acti-vate into destructive cytotoxic T cells. The T_H cells cause dendritic cells to express surface co-stimulatory molecules that are needed for CD8 cell activation.

Responses of the innate immune system are also increased by T_H cells, such as when they activate macro-phages to become more effective killer cells. Cytokines from T_H cells mobilize lymphocytes and macrophages, yet also attract other WBCs. As chemicals are released to bring more cells to defend the body, the immune response increases. The amount of immune elements can then overcome the antigens.

Three primary subsets of T_H cells are import-ant. They may develop during T_H cell differentiation, based on which antigens are present, and the locations in which they are encountered. Another factor is the cytokine exposure of the differentiating T_H cell. The subsets are as follows:

■■ TH1 cells:These cells control most components of cellular immunity. They stimulate inflammation,cause activation of macrophages, and encourage differentiation of cytotoxic T cells.

■■ TH2 cells:They mostly defend against parasitic worms and also promote allergies. They mobilize the eosinophils and cause activation of immune responses depending on B cells and formation of antibodies.

■■ TH17 cells: They releaseIL-17,which helps link adaptive and innate immunity. This substance promotes inflammatory responses that oppose extracellular microbes; these may be the cause of most autoimmune diseases.

Cytotoxic T Cells

Cytotoxic T cells are also calledactivated CD8 cells.They are abbreviated as TC cells, and are the only T cells that directly attack and kill other types of cells. The body is patrolled by TC cells, which circulate through the blood and lymph, as well as through the lymphoid organs. They search for cells with recog-nizable antigens. They primarily target cells infected with viruses, but also attack tissue cells infected with specific intracellular parasites or bacteria, cancer cells, and foreign cells that may have been introduced via organ transplants or blood transfusions.

In order to kill other cells, TC cells must bind to a self-nonself complex on the target cell. Since all body cells display class I MHC antigens, TC cells can destroy all of them that are infected or abnormal. Their attack upon foreign human cells, such as grafted cells, is not easily understood since all of the antigens are nonself. They are probably able to interpret foreign class I MHC antigens as a mixture of self class I MHC protein that is bound to a foreign antigen.

When TC cells recognize their targets, they uti-lize either perforins and granzymes, or they bind to specific membrane receptors on target cells that stimulate apoptosis. Natural killer cells use the same methods to kill target cells, but do not search for foreign antigens displayed on class I MHC proteins. They instead search for other abnormalities such as the lack of class I MHC or the presence of antibodies that coat the target cell. A stressed cell often has a different surface marker that can activate NK cells. Basically, NK cells find abnormal or foreign cells that TC cells cannot find.

Together, NK cells and TC cells closely examine other cells for recognizable markers. This is known as immune surveillance. The NK cells search foridentityflags, and class I MHC proteins actually inhibit NK cellattack. However, the TC cells check these identity flags to determine whether they need to be attacked. For example, foreign antigens stimulate attack by TC cells.

Regulatory T Cells

Regulatory T cells, abbreviated asTReg cells, arerelated to TH cells. They reduce the immune response by either direct contact, or by the release of inhibitory cytokines. These cells are important to prevent auto-immune reactions. They suppress self-reactive lym-phocytes in peripheral tissues. This means that they act outside lymphoid organs. The TReg cells and their subtypes are under active research today. It is hoped that they may be used in the future to cause tolerance to transplanted tissues, and to reduce severity of auto-immune diseases.

1. Explain the actions of lymphocytes in cell-mediated immunity.

2. Differentiate between the ways that lymphocytes and antigen-presenting cells respond to antigens.

3. What are the three major subtypes of T cells?