Chapter 45, Problem 45.1CR

MAKE CONNECTIONS → What forms of signaling activate a helper T cell in immune responses (see Figure 43.16)?

Summary Introduction

To explain: In immune responses, what forms of signaling activate helper T cells.

Concept introduction:

Helper T cells have a crucial role in the regulation of the immune system. Antigen presentation by dendritic cells, macrophages, or B cells activates the helper T cells. Foreign molecule or antigen binds to the antigen receptor of helper T cells. They communicate with antigen-presenting cells through their receptors and CD4. Also, they activate other immune cells by secretion of cytokines. The helper T cell activates both humoral as well as cell mediated immune responses.

Explanation of Solution

Antigen presentation by antigen-presenting cells such as dendritic cells or macrophages activate helper T cells. Antigen-presenting cells have both class I and class II MHC molecules. Class II MHC molecules are used for recognition of antigen-presenting cells. Antigen-presenting cells engulf and degrade pathogens, and they display fragments of antigen—class II MHC molecules complex on the cell surface. Helper T cells bind to this complex through their antigen receptor and CD4 (accessory protein). When helper T cells bind to antigen-presenting cells, cytokine secretion is stimulated. These secreted cytokines and cytokines from helper T cells promote activation and proliferation of helper T cells. The proliferated helper T cells with antigen receptors for this specific antigen fragment complex secrete cytokines that activate B cells (humoral immunity) as well as cytotoxic T cells (cell mediated immunity).