Rouleaux

They occur when the plasma protein concentration is high, and, because of them, the ESR (erythrocyte sedimentation rate) is also increased.

[1] Conditions that cause rouleaux formation include infections, multiple myeloma, Waldenström's macroglobulinemia, inflammatory and connective tissue disorders, and cancers.

Acute-phase proteins, particularly fibrinogen, interact with sialic acid on the surface of RBCs to facilitate the formation of rouleaux.

Rouleaux formations are also adopted by spermatozoa as a means of cooperation between genetically similar gametocytes so as to improve reproductive success through enhanced motility and, therefore, fertilization capacity—e.g., the guinea pig.

According to Smoluchowski aggregation, the kinetics of colloids is based on the assumption that each particle is surrounded by a "sphere influence".

Rouleaux formation on wet smear.
Rouleaux formation on patient vaginal swab wet smear.
Typical mammalian erythrocytes: (a) seen from surface; (b) in profile, forming rouleaux.