Question

Why would giving a patient an antibody that bound to CD4 blocking its interaction with MHCII make the patient less likely to be able to respond to a new bacterial infection?

Answer 1

If CD4 is blocked then the T helper cells will not be activated. Thus no T helper cells will indicate that there is no adaptive immune functions going on. Thus, if a patient with blocked CD4 is infected by n a new bacteria then the immune system of the patient will be less likely to respond to the infection because of his failure in receiving the antigen presented by antigen presenting cell to T helper cells. Though his innate immunity will respond initially but it will not get any signal from the adaptive immune cells.

T helper cells after its activation reaches to the site of infection or inflammation and then it interacts with the macrophages there and gives them a pro survival signals for the macrophages. Generally, without the signal from the T helper cells, the macrophages will die. But due to the pro-survival signal by T helper cells ( IFN gamma is released by them) their half life increases and also their activity increases. Thus , without the T helper cells activation the response towards the a new bacterial infection will be very less likely.