C1 Esterase Inhibitor is a vital human plasma protein that regulates the complement, contact, and intrinsic coagulation systems by inhibiting key serine proteases such as C1r, C1s, factor XIIa, and kallikrein. Deficiency or dysfunction of this inhibitor leads to unregulated activation of the complement cascade and excessive production of bradykinin, causing localized vascular permeability and tissue swelling. This mechanism is directly responsible for hereditary angioedema (HAE), characterized by recurrent, unpredictable swelling episodes in the face, limbs, and airways. Therapeutic plasma-derived or recombinant C1 inhibitors are successfully used to prevent and acutely treat these potentially life-threatening attacks.