What is antigen presentation and why is it critical during the an immune response?

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Explanation:

Sometimes when a pathogenic bacterium gets phagocytosed by a macrophage, the macrophage isn't capable of killing it and needs to get activated.

In order get activated T helper cells (Th1) need to get the signal to release certain chemicals called cytokines to activate the macrophages.

Who gives them this signal that the macrophages need help? Yes, the macrophages themselves by chopping down a piece of the bacteria's antigen and presenting on certain molecules on its surface. This is antigen presentation.

By presenting this antigen, the T helper cells specific to it will get activated and release the cytokines that'll activate the macrophages.

This is also seen with another type of T helper cells (Th2) that activate B cells. And it's also seen in an interaction between a virus-infected cell and a T cytotoxic cell.

This is important for both cell-mediated and humoral immunity.