Fc Receptors Explained: ADCC, ADCP, CDC and the CD32B Brake

An antibody's variable domains decide what it binds. Its Fc decides what happens next — and that is not a detail of formulation, it is half the mechanism. Which Fcγ receptor the constant region engages determines whether an opsonised cell is killed by an NK cell, eaten by a macrophage, lysed by complement, or simply ignored. It also determines whether your isotype control is a control at all. This article walks the Fcγ receptor family, the single kinase cascade they share, the one inhibitory receptor that subtracts from all of it, and the three effector mechanisms they feed — plus FcRn, which does none of the above and yet sets the half-life of the whole molecule.

Affinity is not potency: why CD64 does less than it should

CD64 (FcγRI) binds monomeric IgG with nanomolar affinity — orders of magnitude tighter than the other Fcγ receptors. The intuitive conclusion is that it must dominate therapeutic-antibody activity. It does not, and the reason is the same affinity: at physiological IgG concentrations CD64 is substantially occupied by irrelevant circulating antibody before your molecule arrives.

The low-affinity receptors behave in the opposite way, and it is a feature rather than a limitation. CD16 (FcγRIII) and FcγRIV bind monomeric IgG too weakly to be occupied at baseline, so they only cross-link when many IgG molecules sit close together — which happens on an opsonised target surface and essentially nowhere else. Low affinity plus avidity-dependence equals target-restricted activation. That is why CD16 is the ADCC receptor, and why in-solution affinity measurements can be a poor guide to which receptor will drive the biology.

One inhibitory receptor, co-expressed with the activating ones

CD32B (FcγRIIB) is the only inhibitory member of the family, and the crucial point is that it is not on separate cells. B cells and myeloid cells carry activating and inhibitory FcγRs side by side, so what the cell does when IgG arrives is set by the ratio of engagement, not by the presence of any single receptor.

This is the mechanistic basis of Fc engineering. Afucosylation raises CD16 affinity and shifts the balance towards ADCC; other modifications reduce FcγR binding altogether to make a silent Fc for a molecule where killing would be a liability, such as a checkpoint agonist. On the antigen side it explains why the same clone can be depleting or non-depleting depending on isotype, and why anti-CD19 or anti-CD20 style depletion needs its Fc reported alongside its epitope.

The shared cascade — and the single node that carries it

The activating FcγRs have short cytoplasmic tails and do not signal on their own. They pair with the ITAM-bearing FcεRIγ chain, which LYN phosphorylates and SYK then docks. From SYK the pathway forks: through BTK and PLCγ2 to calcium flux, degranulation and the oxidative burst; and through VAV1 to RAC1 and the actin remodelling that a phagocytic cup requires.

The brake targets the same second messengers rather than the receptor. CD32B’s ITIM recruits SHIP1, an inositol phosphatase that hydrolyses PIP₃ — and PIP₃ is what recruits BTK to the membrane. SHIP1 therefore does not block SYK; it starves the step after it. SHP-1 adds tyrosine dephosphorylation of the proximal machinery. Because activation and inhibition converge, small shifts in receptor occupancy produce large shifts in output — useful when engineering, treacherous when comparing batches.

The same γ chain also serves FcεRI on mast cells and basophils and CD89 (FcαRI) for IgA. One ITAM module, several isotypes.

Three effector mechanisms, three different rate-limiting steps

ADCC — NK-dominated, needs cross-linking

CD16 cross-linking on an NK cell triggers granule release: perforin to breach the membrane and granzyme B to execute. TNF and other cytokines come with it, which is both part of the efficacy and part of the toxicity. If you want to prove ADCC was responsible, deplete the effector: anti-NK1.1 removes the arm outright.

ADCP — macrophage-dominated, and gated outside the Fc axis

Phagocytosis needs the actin arm (CD11b with CD18 as Mac-1, downstream of RAC1) but it is licensed or vetoed by a separate axis entirely: CD47 on the target engaging SIRPα on the macrophage is the dominant "don't eat me" signal, running against surface calreticulin read by LRP1 as "eat me". A cell can be beautifully opsonised and still not be eaten. This is why CD47 blockade combines with opsonising antibodies rather than duplicating them.

CDC — needs geometry, not just occupancy

C1q binds IgG only when several Fc regions are clustered in the right hexameric arrangement on the surface — so CDC is exquisitely sensitive to epitope density and antigen mobility, more so than to affinity. Once C1q is fixed, C1s cleaves C4, C3 amplifies (with Factor B feeding the alternative loop), and C5 splits to give the C5b-9 membrane pore plus C5a acting on C5aR1 — the inflammatory by-product that accounts for a good deal of infusion reaction.

The three mechanisms side by side

The practical implication is that an antibody can be excellent at one of these and useless at another, and a single assay will mislead. A chromium-release ADCC assay says nothing about whether the molecule fixes complement or survives the CD47 checkpoint.

FcRn: the receptor that does not signal

FcRn is grouped with the Fc receptors by name and by ligand, and by nothing else. It does not carry an ITAM, does not activate anything, and does its work inside the cell: IgG taken up by fluid-phase pinocytosis binds FcRn in the acidified endosome, is diverted away from the lysosome, and is released back at the neutral pH of the cell surface. That salvage pathway is what gives IgG its roughly three-week half-life, and it is entirely pH-dependent — which is why Fc mutations that raise FcRn affinity at pH 6 while preserving release at pH 7.4 extend half-life, and why FcRn blockade is a rational strategy in autoantibody-driven disease: it accelerates the clearance of pathogenic IgG without touching complement or FcγR.

What this means for your controls

If a depleting antibody works through Fc, then an isotype-matched control that also binds FcγR is not a negative control — it is a second experimental arm. Three practical consequences. First, report the isotype, not just the clone: mouse IgG2a engages FcγRIV strongly and depletes; IgG1 largely does not. Second, block the receptor rather than relying on the isotype: anti-CD32/CD16 (clone 2.4G2) is the standard Fc block, and anti-CD16-2 removes the FcγRIV arm specifically. Third, remove one effector cell at a time — anti-NK1.1 for ADCC, anti-CD11b and anti-CD18 for the myeloid arms — and see which loss abolishes the effect. Endotoxin matters here too: LPS contamination activates myeloid cells directly, so a contaminated blocking antibody can increase the very effector function you were trying to remove.

Frequently asked questions

Why does CD64 contribute less than its affinity suggests?

Because affinity that high is a liability at baseline. CD64 binds monomeric IgG, so it is substantially occupied by circulating antibody before a therapeutic molecule arrives. The low-affinity receptors CD16 and FcγRIV stay free until IgG is multimerised on a target, which makes their activation target-restricted.

What actually distinguishes ADCC from ADCP?

The effector cell and the rate-limiting step. ADCC is NK-dominated and limited by CD16 cross-linking density, ending in perforin and granzyme B. ADCP is macrophage-dominated, limited by actin remodelling through CD11b/CD18, and vetoed by the CD47–SIRPα axis regardless of how well the target is opsonised.

Is an isotype control really not a control?

Not on its own, if the mechanism runs through Fc. An isotype-matched irrelevant antibody still engages FcγR and FcRn; what it does not do is bind your antigen. That is a useful comparison but it does not isolate the Fc contribution. To do that, block the receptor (anti-CD32/CD16) or remove the effector cell (anti-NK1.1).

Why do some antibodies fix complement well and others not at all?

Geometry. C1q requires several Fc regions clustered in a hexameric arrangement, so CDC depends on epitope density, epitope height above the membrane and antigen mobility — not simply on how tightly the antibody binds. Two antibodies with identical affinity for the same antigen can differ completely in CDC.

What does SHIP1 actually inhibit?

SHIP1 hydrolyses PIP₃, and PIP₃ is what recruits BTK to the membrane. So SHIP1 does not switch off SYK — it starves the step immediately downstream. That is why the inhibitory signal from CD32B is graded rather than absolute, and why the activating-to-inhibitory ratio is the variable that matters.

Does FcRn have anything to do with effector function?

No, and that is the point worth remembering. FcRn recycles IgG out of the endosome and sets its half-life; it carries no ITAM and triggers no killing. It is why Fc engineering splits into two independent problems: how long the molecule lasts, and what it does while it is there.

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