The loose-fitting sac around the heart is the:

Capillary walls consist of a single layer of epithelial cells, and they exchange substances in the blood for substances in the tissue fluid surrounding body cells.

This single layer of cells is called the endothelium and it forms the barrier between the blood and the interstitial fluid.

The endothelium can be either continuous or fenestrated, depending on the tissue type and function.

The capillaries are very thin and allow red blood cells to flow through them single file.

The capillaries also have a layer of a glycoprotein called the glycocalyx that covers their luminal surface.

Choice B. False is wrong because it contradicts the definition and structure of capillaries.

Capillaries are not made of multiple layers of cells, nor do they prevent the exchange of substances between the blood and the tissue fluid.

The correct answer is choice B. True.

As a result of the allergen-antibody reaction, mast cells release histamine.

This is because when mast cells are exposed to an allergen that binds to IgE antibodies attached to their FcɛRI receptors, they release histamine and other inflammatory mediators.

Histamine causes vasodilation, increased capillary permeability, mucus secretion, and smooth muscle contraction, which result in allergic symptoms such as itching, swelling, sneezing, and wheezing.

Choice A is wrong because it is the opposite of the correct answer.

Mast cells do release histamine in response to allergen-antibody reactions. Some additional sentences are:

Normal ranges of histamine levels vary depending on the tissue and the method of measurement, but they are usually low in healthy individuals and elevated in allergic conditions.

Histamine can be degraded by enzymes such as diamine oxidase and histamine N-methyltransferase, or inhibited by drugs such as antihistamines.