When we have fever our immune cells in our body, produces a large amount of WBC's in our body to fight against the pathogen or parasite. My question is after the immune response has finished fighting the pathogens, what happens to the excess cells and ABs that presumably are no longer needed? Will they disintegrate themselves, is there a final step in the immune response to dispose of these cells, or are excess of these cells and proteins present as if they are not in excess (they are filtered no more so than when at normal levels)?

Fever isn't actually caused by an increase in WBCs in the body. Read the pathophysiology section of this article:

WBCs are phagocytosed by other WBCs when they reach the end of their life cycles and the proteins etc in them are recycled, as with all other cells and antibodies.