What causes blood pressure to decrease during an acute allergic reaction?

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During an acute allergic reaction, blood pressure decreases primarily due to vasodilation and fluid shifting. When the body encounters an allergen, it releases histamines and other inflammatory substances that cause blood vessels to dilate. This vasodilation increases the capacity of the vascular system, allowing more blood to fill the dilated vessels, which can lead to a drop in blood pressure.

Additionally, as fluid shifts out of the blood vessels and into the surrounding tissues—often resulting from increased permeability of the blood vessel walls—there is a decrease in the effective circulating blood volume. This further contributes to the overall reduction in blood pressure. Hence, the combined effects of vasodilation and the translocation of fluid can lead to symptoms such as hypotension, which is a common manifestation in acute allergic reactions like anaphylaxis.

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