M1 vs M2 Macrophage: Differences and Naming

M1 vs M2 macrophage differences: stimuli, function, and why Murray 2014 recommends naming by activator, plus mouse-human and culture caveats.

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Last updated 29 October 2026

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How do M1 (classically activated), M2 (alternatively activated) compare side by side?

The table below compares M1 (classically activated), M2 (alternatively activated) across 12 procurement-relevant dimensions, from original meaning of the label through source.

Side-by-side comparison

Dimension	M1 (classically activated)	M2 (alternatively activated)
Original meaning of the label	Classically activated macrophages. M1 is the older shorthand for this activation state.	Alternatively activated macrophages. M2 covers several subtypes rather than one state.
Prototypical activating stimuli	IFN-γ and LPS (Martinez 2008).	IL-4 is one of the activators Murray 2014 uses for naming: M(IL-4). Other activators it names include immune complexes (Ig), IL-10 and glucocorticoids (GC).
Functional profile described	Strong microbicidal activity; supports Th1 responses.	Supports Th2-associated functions and helps resolve inflammation.
Subtypes	Treated as a single classical state in the two-type model.	Several subtypes. Murray 2014 recommends naming by activator, such as M(IL-4), M(Ig), M(IL-10) and M(GC), instead of M2a/M2b-style subtypes.
Tumor-associated macrophages	Martinez 2008 places tumor-associated macrophages on the alternative side, not the classical one.	Tumor-associated macrophages share an alternative-like activation profile and play a harmful, pro-tumoral role (Martinez 2008).
What the label tells you versus hides	Tells you the broad direction (IFN-γ/LPS-type, pro-inflammatory). Hides which activator, which cell source and which time point.	Tells you only "not classical". Hides which of several activators was used and which subtype is meant. Murray 2014: the two-type theory "requires rethinking".
Identifying by markers	Murray 2014 favors combinations of markers, not a single readout. Arg1 is also induced in M1-spectrum macrophages.	Arg1 alone does not identify M2 for the same reason. A combined panel is needed. For a lung population in Th2 disease, Murray 2014 gives the example set Arg1hi, Retnlahi, pSTAT6+, pSTAT1-.
Mouse versus human	Many markers used for murine macrophages have not translated to human macrophages (Murray 2014).	Same caveat. Murray 2014 notes no study has systematically compared responses of blood monocyte-derived macrophages from mice and humans.
In vitro culture effects	Murray 2014 Table 1: medium (DMEM versus RPMI) has substantial effects on growth rate, development and activation status; plastic type also affects growth and activation.	Same factors apply. L cell-conditioned media contains variable amounts of type I interferons, so purified recombinant CSF-1 is preferred.
Timing of analysis	Murray 2014: reporting the time points of ex vivo macrophage isolation and analysis is mandatory.	Same requirement.
When to use each label	Use "M1" as shorthand only when the broad classical, IFN-γ/LPS-type state is meant, and state the activator, e.g. M(IFN-γ) or M(LPS).	Prefer a named activator such as M(IL-4) in a methods section; use "M2" only for the broad alternative-activation concept.
Source	Murray PJ et al., "Macrophage activation and polarization: nomenclature and experimental guidelines", Immunity 2014;41(1):14-20 (PMC4123412); Martinez FO, Sica A, Mantovani A, Locati M, "Macrophage activation and polarization", Front Biosci 2008;13:453-61 (PubMed 17981560). Both read 2026-10-29.	Murray PJ et al., "Macrophage activation and polarization: nomenclature and experimental guidelines", Immunity 2014;41(1):14-20 (PMC4123412); Martinez FO, Sica A, Mantovani A, Locati M, "Macrophage activation and polarization", Front Biosci 2008;13:453-61 (PubMed 17981560). Both read 2026-10-29.

Common questions

Common questions about M1 (classically activated) vs M2 (alternatively activated)

What are M2 macrophages?

M2 macrophages are the alternatively activated macrophages in the two-type model. They support Th2-associated functions and help resolve inflammation (Martinez 2008). M2 covers several subtypes, and Murray 2014 recommends naming by activator, such as M(IL-4), M(Ig), M(IL-10) or M(GC).

What is the difference between M1 and M2 macrophages?

M1 macrophages are classically activated, typically by IFN-γ and LPS, with strong microbicidal activity and support for Th1 responses. M2 macrophages are alternatively activated, support Th2-associated functions and help resolve inflammation (Martinez 2008). Murray 2014 argues this two-type view requires rethinking.