Opsonin

[1] Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. eaten) by phagocytes (cells that specialise in phagocytosis, i.e. cellular eating).

They concluded that: “We have here conclusive proof that the blood fluids modify the bacteria in a manner which renders them a ready prey to the phagocytes.

We may speak of this as an “opsonic” effect (opsono - I cater for; I prepare victuals for), and we may employ the term “opsonins” to designate the elements in the blood fluids which produce this effect.”[3] Subsequent research found two main types of opsonin in blood that opsonised bacteria: complement proteins[4] and antibodies.

[5] Both IgM and IgG undergo conformational change upon binding antigen that allows complement protein C1q to associate with the Fc region of the antibody.

C3d, a cleavage product of C3, recognizes pathogen-associated molecular patterns (PAMPs) and can opsonize molecules to the CR2 receptor on B cells.

[7] In the alternative pathway of complement activation, circulating C3b is deposited directly onto antigens with particular PAMPs, such as lipopolysaccharides on gram-negative bacteria.

[8] Members of the pentraxin family can bind to apoptotic cell membrane components like phosphatidylcholine (PC) and phosphatidylethanolamine (PE).

Collectin molecules such as mannose-binding lectin (MBL), surfactant protein A (SP-A), and SP-D interact with unknown ligands on apoptotic cell membranes.

When bound to the appropriate ligand these molecules interact with phagocyte receptors, enhancing phagocytosis of the marked cell.

[6] As part of the late stage adaptive immune response, pathogens and other particles are marked by IgG antibodies.