Sialoadhesin is a cell adhesion molecule

Cell adhesion molecule

Cell Adhesion Molecules are proteins located on the cell surface involved with the binding with other cells or with the extracellular matrix in the process called cell adhesion....

 found on the surface of certain cells of the immune system

Immune system

An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 called macrophage

Macrophage

Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

s. It is found in especially high amounts on macrophages of the spleen, liver, lymph node, bone marrow, colon, and lungs. Also, in patients suffering from rheumatoid arthritis, the protein has been found in great amounts on macrophages of the affected tissues. It is defined as an I-type lectin

Lectin

Lectins are sugar-binding proteins that are highly specific for their sugar moieties. They play a role in biological recognition phenomena involving cells and proteins. For example, some viruses use lectins to attach themselves to the cells of the host organism during infection...

, since it contains 17 immunoglobulin (Ig) domains

Immunoglobulin domain

The immunoglobulin domain is a type of protein domain that consists of a 2-layer sandwich of between 7 and 9 antiparallel β-strands arranged in two β-sheets with a Greek key topology....

 (one variable domain and 16 constant domains), and thus also belongs to the immunoglobulin superfamily

Immunoglobulin superfamily

The immunoglobulin superfamily is a large group of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells. Molecules are categorized as members of this superfamily based on shared structural features with immunoglobulins ; they all possess a...

 (IgSF). Sialoadhesin binds to certain molecules called sialic acid

Sialic acid

Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone. It is also the name for the most common member of this group, N-acetylneuraminic acid...

s. During this binding process a salt bridge (protein)

Salt bridge (protein)

Salt bridges fall into the broader category of noncovalent interactions. A salt bridge is actually a combination of two noncovalent interactions: hydrogen bonding and electrostatic interactions . This is most commonly observed to contribute stability to the entropically unfavorable folded...

 is formed between a highly conserved arginine residue (from the v-set domain to the 3'-sialyllactose) and the carboxylate group of the sialic acid. Since sialoadhesin binds sialic acids with its N-terminal IgV-domain, it is also a member of the SIGLEC

SIGLEC

Siglecs, short for sialic acid binding Ig-like lectins are cell surface receptors and members of the immunoglobulin superfamily that recognize sugars. Their ability to recognize carbohydrates using an immunoglobulin domain places them in the group of I-type lectins...

 family. Alternate names for sialoadhesin include siglec-1 and CD169 (cluster of differentiation

Cluster of differentiation

The cluster of differentiation is a protocol used for the identification and investigation of cell surface molecules present on white blood cells, providing targets for immunophenotyping of cells...

 169).

Sialoadhesin predominately binds neutrophils, but can also bind monocyte

Monocyte

Monocytes are a type of white blood cell and are part of the innate immune system of vertebrates including all mammals , birds, reptiles, and fish. Monocytes play multiple roles in immune function...

s, natural killer cell

Natural killer cell

Natural killer cells are a type of cytotoxic lymphocyte that constitute a major component of the innate immune system. NK cells play a major role in the rejection of tumors and cells infected by viruses...

s, B cell

B cell

B cells are lymphocytes that play a large role in the humoral immune response . The principal functions of B cells are to make antibodies against antigens, perform the role of antigen-presenting cells and eventually develop into memory B cells after activation by antigen interaction...

s and a subset of cytotoxic T cell

Cytotoxic T cell

A cytotoxic T cell belongs to a sub-group of T lymphocytes that are capable of inducing the death of infected somatic or tumor cells; they kill cells that are infected with viruses , or are otherwise damaged or...

s by interacting with sialic acid molecules in the ligand

Ligand

In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s on their surfaces.