Pathogens that infect the human body replicate either inside cells (such as viruses) or extracellularly, in the blood or in the extracellular spaces in tissues

Question

A. Identify (i) the class of T cells that are stimulated by intracellular pathogens, (ii) their co-receptor, (iii) the MHC molecule used for recognition of antigen and (iv) the T-cell effector function.
B. Repeat this for the classes of T cells that are stimulated by extracellular pathogens. For the purposes of this question, count those pathogens (such as mycobacteria) that can survive and live inside intracellular vesicles after being taken up by macrophages as extracellular pathogens.

What will be an ideal response?

Answer 1

A. (i) Pathogens that are propagating freely within cells (for example viruses) are eradicated by the actions of cytotoxic T cells. (ii) Cytotoxic T cells express a glycoprotein called CD8, a T-cell co-receptor that interacts with (iii) MHC class I on antigen-presenting cells. (iv) Once activated, cytotoxic T cells kill cells infected with the pathogen, which are displaying pathogen peptides on MHC class I molecules, and thereby inhibit further replication of the pathogen and infection of neighboring cells.
B. (i) Pathogens that reproduce in extracellular spaces, for example encapsulated bacteria such as Streptococcus pneumoniae, are eradicated after the activation of other cell types by helper T cells, namely the classes TH1 and TH2. (ii) TH1 and TH2 cells express a glycoprotein called CD4, a T-cell co-receptor that interacts with (iii) MHC class II molecules on antigen-presenting cells. (iv) TH1 cells activate macrophages that are displaying pathogen peptides (derived from phagocytosed pathogen) on MHC class II molecules on their surface. This stimulates increased phagocytosis by the macrophage and destruction of pathogens inside phagolysosomes. Activated macrophages also secrete inflammatory mediators that have an important part in eradicating the infection by helping to induce inflammation which recruits phagocytic cells and effector lymphocytes to the site of infection. TH1 cells also induce switching of B cells to certain antibody isotypes. TH2 cells activate B cells displaying antigen-derived peptides on MHC class II molecules, resulting in the differentiation of the B cells into plasma cells and the production of antibodies that remove the extracellular pathogen or its toxins as a result of neutralization, opsonization, and complement activation.