Question

Please give a detailed response and describe how T cells coordinate the specific humoral immune response. Then describe how T cells coordinate the specific cell mediated immune response. For both, be sure to discuss all receptors involved, interleukins, all cell interactions and cell types.

Answer 1

Naive T cells are activated to produce armed effector T cells the first time they encounter their specific antigen in the form of a peptide:MHC complex on the surface of an activated antigen-presenting cell (APC).

The most important antigen-presenting cells are the highly specialized dendritic cells, whose only known function is to ingest and present antigen.

Tissue dendritic cells ingest antigen at sites of infection and are activated as part of the innate immune response. This induces their migration to local lymphoid tissue and their maturation into cells that are highly effective at presenting antigen to recirculating T cells. These mature dendritic cells are distinguished by surface molecules, known as co-stimulatory molecules, that synergize with antigen in the activation of naive T cells.

Macrophages provide a first line of defense against infection, can also be activated to express co-stimulatory and MHC class II molecules. This enables them to act as antigen-presenting cells, although they are less powerful than dendritic cells at activating naive T cells.

Dendritic cells, macrophages, and B cells are often known as professional antigen-presenting cells.

Effector T cells, fall into three functional classes that detect peptide antigens derived from different types of pathogen.

Peptides from intracellular pathogens that multiply in the cytoplasm are carried to the cell surface by MHC class I molecules and presented to CD8 T cells. These differentiate into cytotoxic T cells that kill infected target cells. Peptide antigens from pathogens multiplying in intracellular vesicles, and those derived from ingested extracellular bacteria and toxins, are carried to the cell surface by MHC class II molecules and presented to CD4 T cells. These can differentiate into two types of effector T cell, called TH1 and TH2.

Pathogens that accumulate in large numbers inside macrophage and dendritic cell vesicles tend to stimulate the differentiation of TH1 cells, whereas extracellular antigens tend to stimulate the production of TH2 cells. TH1 cells activate the microbicidal properties of macrophages, and induce B cells to make IgG antibodies that are very effective at opsonizing extracellular pathogens for uptake by phagocytic cells. TH2 cells initiate the humoral immune response by activating naive antigen-specific B cells to produce IgM antibodies. These TH2 cells can subsequently stimulate the production of different isotypes, including IgA and IgE, as well as neutralizing and/or weakly opsonizing subtypes of IgG.