Efferocytosis: Macrophage Clearance Race

Tissue Parameters

Macrophages 8

Apoptosis rate 0.6 cells/s

Chemotactic sensitivity 1.0×

Secondary-necrosis window 9 s

Live Readouts

Cleared (silent)

Secondary necrosis

Mean macrophage load

Clearance efficiency

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How it works

Efferocytosis is the continual, largely silent phagocytic clearance of apoptotic ("dying-by-design") cells by macrophages. It is a race against time: a cell that dies apoptotically must be engulfed before it loses membrane integrity and undergoes secondary necrosis, which spills damage-associated molecular patterns (DAMPs) and triggers inflammation — the process this failing is implicated in atherosclerosis and lupus.

Eat-me signal (PS exposure):
  E(t) = 1 − e^(−t/τ_PS)      (phosphatidylserine flips outward)

Engulfment probability per Δt:
  P = 1 − e^(−k·E·Δt)         (gated: only if macrophage load L < 1)

Macrophage phagocytic load:
  dL/dt = −L/τ_digest         (decays as phagosome processes cargo)
  L += ΔL_bite on each successful engulfment

Secondary necrosis trigger:
  t_age > T_window  AND  cell not yet engulfed

Find-me signals (modeled as a chemotactic field, real biology: LPC, ATP/UDP, S1P) diffuse from apoptotic cells; macrophages steer up the combined gradient of nearby sources.
Eat-me signal — exposed phosphatidylserine (PS) ramps up with time-since-death (green → amber). Recognition (via receptors like MerTK) only becomes reliable once E(t) crosses a threshold.
Finite capacity — each macrophage can only digest so much cargo at once (finite phagolysosome throughput); load saturates near 1 and blocks further engulfment until it decays.
Secondary necrosis (red flash) — a cell not cleared inside the window lyses and releases DAMPs, an inflammatory outcome efferocytosis exists to prevent.

Macrophage Fresh apoptotic (low PS) PS-exposed (eat-me active) Secondary necrosis (DAMPs)

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Efferocytosis: Macrophage Clearance Race Against Secondary Necrosis