Histamines in the human body cause the contractions of muscle tissue in the lungs, uterus and stomach, according to Encyclopaedia Britannica. Histamines also dilate blood vessels, accelerate the heart rate and increase stomach acid production. This protein serves as a neurotransmitter between the nervous system and other cells in the body. Systemically, the most obvious sign of a histamine reaction is inflammation.
Encyclopaedia Britannica indicates histamines activate during an immune response within the body. At an injury site, histamines cause more permeability in cell membranes, which allows fluids, white blood cells and blood plasma proteins to arrive more quickly than normal. Special cells in the body called mast cells release histamines in injured tissue to begin the immune system response.
An allergic reaction occurs in the human body when histamines believe harmless substances such as dust or pollen are invading germs, according to the Biology Department at Davidson College. Histamines increase swelling in bronchial tubes within the lungs during asthmatic reactions when external stimuli enter the organs. Medicines known as antihistamines help control these allergic responses. Allergies that cause histamine reactions are not normal because dust and pollen are not dangerous to the human immune system.
Histamines are released in large amounts during an allergic reaction. Massive amounts of histamines released in the body cause systemic reactions such as sneezing, coughing, runny nose and itchy eyes, explains IMS Health Incorporated.