Receptor mediated endocytosis is the ingestion of molecules bound to the surface receptors of the cell into endosomes. This is how antigens bound to the receptors of B lymphocytes, for example, succeed in entering the cell.

Receptors: The expression of receptors for antigens on the cell surface is the defining property of lymphocytes. Every lymphocyte bears a receptor with a unique structure. The gene for this receptor is created by the rearrangement of receptor segments during the lymphocyte maturation.

Rejection reaction: The destruction of foreign transplanted tissue by aggressive lymphocytes is called a rejection reaction. A distinction is made between the initial (primary) rejection reaction and the following (secondary) rejection reactions against transplants from the same donor or from a person related to the first donor. Secondary rejection reactions are generally much more pronounced than primary reactions.

Retroviruses are widespread. HIV, for example, is one of the retroviruses. The genetic information is stored in the form of single-strand RNA.

Reverse transcriptase is an essential enzyme of retroviruses. It transcribes the RNA genome of these viruses into DNA which is then integrated into the genome of the host cell. Reverse transcriptase is also an important tool in molecular biology. It can be used to transcribe RNA into cDNA for cloning.

Rhesus blood type antigens: The Rhesus blood type antigens (RH antigens) are antigens in the membrane of the red blood cells which are also present in Rhesus monkeys.

Rheumatoid arthritis: Rheumatoid arthritis is a very common inflammatory disease of the joints which probably results from an autoimmune reaction. It goes hand in hand with the production of the so-called rheumatoid factor, an IgM anti-IgG antibody which can also be produced as part of normal immune responses.

RNA (Ribonucleic acid) is also a carrier of genetic information. In contrast to DNA, RNA is generally single strand.