How does a decrease in Helper T cells impair all aspects of the immune response? I got this question wrong on one of the KA passages and wasn't sure why that would be the case? Could someone elaborate on why this is true?
Helper T-cells (CD4) bind to MHCII receptors, which are present on antigen-presenting cells. These include macrophages and B cells. When the CD4 cells bind to the MHCII, with an antigen bound, it will activate B cells which would allow the B cells to proliferate into plasma and memory cells (humoral immunity). Also, CD4 cells can also interact with cytotoxic T-cells (CD8) to activate them so that CD8 cells can kill infected cells (cell-mediated immunity). Without helper T cells, humoral and cell-mediated immunity would be greatly impaired. Due to this, patients suffering from AIDS have a greatly diminished helped T cell count so they are very susceptible to become sick from many pathogens that would usually be harmless.
I get how CD4+ cells release cytokines to activate B-cells for humoral immunity and cytotoxic T cells (CD8+) for cell-mediated response.
I feel like many online sources say that CD4+ activate macrophages. I thought the antigens enter the macrophage, and therefore the MHCII receptors activate CD4+ cells which causes them to release cytokines? (like the picture above.) Any clarification would be appreciated.
What you said is right, macrophages is one way to activate the Helper T cells, as shown in your diagram. However, any antigen presenting cells that has MHCII receptors in addition to an antigen could potentially activate a helper T cell. The pathways are a lot more complex and I wouldn't worry too much about it, just remember the interdependence of the certain cells necessary for humoral and cell mediated immunity.
I just don't get how all aspects of the immune response would be affected. There would still be the nonspecific defense of the body. You have your fever, inflammation, and macrophages like neutrophils and eosinophils. Seems a bit extreme to say all or am I wrong here?
But don't CD8 respond to nucleated cells only? Helper T cells would only promote faster maturation of CD8 cells and stimulate production. It is not like cytotoxic T cells become activated thru them (helper T). Isn't this true?