Why do we get a bulge on the skin after a mosquito bite?

Stings of insect group animals like mosquito cause skin lesions by direct effects of the insect parts or secretions which cause irritation. When the insect parts or secretions are retained for some time they tend to cause hypersensitivity responses. The immediate itching effect on the site of the bite is the appearance of urticaria or inflamed papules. Histologically the lesion shows a wedge shaped per-vascular infiltrate of the lymphocytes, histocytes and eosinophils within the dermis. The first event of the inflammation is an increased blood flow to the bite area. This results mainly due to the arteriolar dilation. Another event is the increased vascular permeability which results in the accumulation of protein rich extra vascular fluid.

The major chemical mediator of inflammation is the histamine. It is widely distributed in the tissue, the richest source being the mast cells that are normally present in the corrective tissues adjacent to the blood vessels. Preformed histamine is present in mast cell granules and is released by mast cell de-granulation process which in response to the stimulus caused due to irritation at the site of the bite. This histamine causes dilation of the arterioles and increase vascular permeability of venules. This in turn causes venular endothelial contraction and widening of the interendothelial cell Junctions, where the extra-vascular fluid accumulates causing, inflammation.