Editing CD32 (section)

== Functions and locations ==

=== ''CD32A'' ===
CD32A is an activating subtype of CD32 that can be found on a variety of immune cells - notably, CD32A is found on [[platelet]]s, [[neutrophil]]s, [[macrophage]]s, and [[dendritic cell]]s (DCs). On platelets, it is known to aid in the internalization of IgG-[[Opsonin|opsonized]] ''[[Escherichia coli]]'', and it is more generally implicated in mediating bacterial-activated platelet responses.<ref name=":3" /> CD32A also plays an important role in platelet activation, adhesion, and aggregation in response to injured blood vessels.<ref name=":1" /> When bound to an IgG immune complex, the cytosolic ITAM can promote [[Phagocytosis|phagocytic activity]] and [[Cytokine|cytokine secretion]] in neutrophils and macrophages.<ref name=":0" /> CD32A is known to aid in the activation of [[Clathrin|clathrin coat]]-mediated endocytosis on various cell types. On DCs, CD32A plays an important role in maturation and the upregulation of [[B7 (protein)|costimulatory molecules]] on the cell surface, strengthening the DC's ability to [[Antigen presentation|present antigen]] to T cells. CD32A activation is necessary and sufficient to produce T cell anti-tumor cellular immunity. CD32A is also linked to [[autoimmunity]]; for example, the production of antibodies against [[platelet factor 4]] (PF4) bound to CD32A is linked to the development of [[heparin-induced thrombocytopenia]].<ref name=":3" />

CD32A is also found on [[Langerhans cell|Langerhans]] cells, [[mast cell]]s, [[basophil]]s, [[eosinophil]]s, [[monocyte]]s, [[megakaryocyte]]s, and a subpopulation of activated [[T helper cell|CD4+ T cells]]. CD32A is unique to primates.<ref name=":1" />

=== ''CD32B'' ===
CD32B is an inhibitory surface receptor that is part of a large population of [[B cell]] co-receptors, which act to modulate signaling.<ref name=":1" /> Activated CD32B has the ability to cross-link with [[B-cell receptor|B cell receptors]] (BCRs), which increases the threshold for B cell activation and downregulates antibody production in the presence of IgG.<ref name=":2" /> This feedback loop lowers the production of IgG by B cells when there is a surplus of IgG in the body. CD32B is also found on the surface of [[follicular dendritic cells]] (FDCs), which utilize CD32B for the retention and recycling of immune complexes that they later present to B cells.<ref name=":1" /><ref name=":4">{{cite journal | vauthors = Hill DL, Schofield L, Wilson DW | title = IgG opsonization of merozoites: multiple immune mechanisms for malaria vaccine development | journal = International Journal for Parasitology | volume = 47 | issue = 10–11 | pages = 585–595 | date = September 2017 | pmid = 28668325 | doi = 10.1016/j.ijpara.2017.05.004 }}</ref> Thus, CD32B plays an important role in both antibody and [[Immunological memory|memory immune responses]].<ref name=":1" />

The balance between CD32B and its activating counterparts is crucial to appropriate cell function. Having too little CD32B has been associated with dysregulated antibody function, as well as increased antibody-dependent inflammatory cell responses.<ref name=":1" /> Some individuals inheriting mutated, inactivate CD32B genes have a reduced risk of contracting malaria; this is attributed to an enhancement of FcR-dependent phagocytic functions.<ref name=":4" /> CD32B imbalance is also associated with autoimmunity. CD32B-deficient mice have been found to be more susceptible to immune-complex-mediated autoimmunity. Likewise, [[systemic lupus erythematosus]] (SLE) in humans is associated with a decrease in CD32B on the surface of [[memory B cell]]s. A decrease on dendritic cells is often found in patients with [[rheumatoid arthritis]].<ref name=":0" /><ref name=":2" /> The therapeutic usage of monoclonal antibodies against CD32B can be effective for inducing cytotoxicity against [[B-cell lymphoma|B cell lymphoma]] cells.<ref name=":1" />

CD32B is also found on basophils, neutrophils, monocytes, and macrophages.<ref name=":0" />

==== Non-immune system locations ====
CD32B can be found on airway [[smooth muscle]] cells, as well as [[liver sinusoidal endothelial cell]]s and [[salivary gland]] epithelial cells.<ref name=":1" /><ref name=":4" />

=== ''CD32C'' ===
CD32C is expressed in ~20% of the human population, and is not well-understood.<ref name=":0" /> It can be found on B cells and [[Natural killer cell|natural killer]] (NK) cells. When expressed, CD32C plays an important role in the activation of [[Antibody-dependent cellular cytotoxicity|antibody-dependent cell cytotoxicity]] (ADCC).<ref name=":1" /> Animal studies have linked CD32C to augmentation of pathological inflammatory responses.<ref name=":1" />