Activation of T-cell immune response, illustration. Array of proteins interacting on the surface membranes of a T-cell (across bottom) and an antigen presenting cell (APC, across top). These proteins are involved in both the activation and...

Histamine (red) binding to receptor, computer illustration. Histamine is responsible for the symptoms of an allergic reaction. It causes pain and itching, dilates capillaries and makes them more permeable, leading to red skin and swelling.

Histamine (red) binding to receptor, computer illustration. Histamine is responsible for the symptoms of an allergic reaction. It causes pain and itching, dilates capillaries and makes them more permeable, leading to red skin and swelling.

Transmembrane proteins. Illustration of five transmembrane proteins with their ligands (left to right): a potassium channel, a delta-opioid receptor, an LDL (low-density lipoprotein) receptor, an acetylcholine receptor, and a histamine receptor. Each...

Cell membrane with receptors. Computer illustration showing molecules approaching receptors on the surface of a cellular membrane.