The helper T cells are activated by the antigens present in the antigen-presenting cells. They stimulate plasma B cells to produce antibodies.
The two types of immune responses generated by the immune system of the body are non-specific and specific immune responses. Natural flora, physical barriers, chemical barriers, phagocytes, histamine, inflammation, fever, and complement proteins are the components of the nonspecific immune response. Humoral and cell-mediated immunity are the two components of specific immune response. Humoral immunity is generated by circulating antibodies. The cell-mediated immunity is generated by the antigen-specific T cells.
Key Areas Covered
Key Terms: Antibodies, B cell Activation, Cell-Mediated Immunity, Helper T Cells, T Cell Receptor (TCR)
What is Cell-Mediated Immunity
Cell-mediated immunity is a type of adaptive immunity, which is mediated by antigen-specific T cells. T cells are a type of lymphocytes produced in the bone marrow. They are matured in the thymus. The two types of T cells are helper T cells and cytotoxic T cells. Helper T cells recognize antigens presented by the antigen-presenting cells and activate B cells to produce antigens. The cytotoxic T cells destroy the infected cells by inducing apoptosis. The cell-mediated immunity is shown in figure 1.
How Do Helper T Cells Activate B Cells
Activated B cells are the type of lymphocytes involved in the production of antibodies specific to a particular antigen. B cells are activated by helper T cells. Helper T cells are activated by the recognition of antigens presented by antigen-presenting cells. Antigen-presenting cells (APCs) such as dendritic cells, macrophages, etc. have to process antigens and present them on their cell membrane along with major histocompatibility complexes (MHC). T helper cells recognize these antigens with the help of T cell receptors and become activated. Once T helper cells encounter an antigen, they proliferate and differentiate into armed effector cells, which produce proteins to activate B cells. The activated B cells produce antibodies against the specific antigen. The helper T cell-dependent B cell Activation is shown in figure 2.
Generally, T cells have two components that help in the activation of T cells;
i) lymphokines, which act as growth and differentiation factors for B cells
ii) additional signals required for B cell contact and enabling B cells to respond to lymphokines.
The cell-to-cell contact occurs through T cell receptors (TCR) of T helper cells and MHC II complexes of B cells.
Cell-mediated immunity is a type of adaptive immunity in which the activated T helper cells activate the B cells for the production of specific antibodies to an antigen. T helper cells are activated by recognizing antigens on the antigen presenting cells. Upon activation, they proliferate and present antigens to the B cells through TCR-MHC receptor complexes. Upon activation, B cells produce antibodies for that particular antigen.
1. Janeway, Charles A, and Jr. “B-Cell Activation by Armed Helper T Cells.” Immunobiology: The Immune System in Health and Disease. 5th Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.
1. “2218 Clonal Selection and Expansion of T Lymphocytes” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia
2. “T-dependent B cell activation” By Altaileopard – Own work (Public Domain) via Commons Wikimedia