Ocrelizumab is an intravenously administered glycosylated IgG1 recombinant humanized anti-CD20 monoclonal antibody that, following infusion, enters systemic circulation immediately and distributes widely throughout the vascular and extravascular spaces. Its therapeutic effect is anchored in the selective depletion of CD20-expressing immune cells, beginning with a rapid loss of peripheral B cells after each dose and followed by slow reconstitution once dosing stops.

The primary molecular target, CD20, is a transmembrane protein expressed on most cells of the human B-cell lineage but not on stem cells, pro-B cells, or differentiated plasma cells — a distribution that restricts depletion to pre-B through mature B cells while leaving CD20-negative early precursors and antibody-secreting plasma cells intact. Although its endogenous function is incompletely understood, CD20 has been linked to the regulation of B-cell growth, activation, and differentiation, together with transmembrane calcium flux. Antibody engagement is itself a signaling event: the intracellular region of CD20 is phosphorylated upon antibody binding, coupling extracellular recognition to downstream cellular behavior.

At the binding level, ocrelizumab targets the large extracellular loop of CD20, with contact residues reported at positions 165–180. Structurally, rituximab and ocrelizumab bind the same epitope on the large loop of CD20, and crystallographic work shows a Fab–Fab interface in the ocrelizumab–CD20 peptide lattice nearly identical to that of rituximab, consistent with homotypic Fab interactions on the intact receptor surface. As a humanized IgG1, ocrelizumab is less immunogenic than the chimeric rituximab scaffold, which reduces the risk of anti-drug antibody formation and infusion reactions.

Once the antibody has opsonized CD20+ targets, depletion proceeds through overlapping Fc-dependent effector pathways, with Antibody-dependent cellular cytotoxicity as the dominant mechanism. ADCC is complement-independent and is initiated when the Fc region of the bound antibody engages FcγRIIIa on NK cells, triggering downstream signaling that culminates in the release of cytolytic mediators granzyme B and perforin and B-cell lysis. Relative to rituximab, ocrelizumab exhibits two- to fivefold greater ADCC activity, associated with enhanced binding to low-affinity variants of FcγRIIIa.

Complement-dependent cytotoxicity contributes as a secondary route. As a type I anti-CD20 antibody, ocrelizumab clustering stabilizes CD20 on lipid rafts, promoting stronger C1q binding and complement activation. The Fc terminus harbors a C1q-binding site that activates the classical complement pathway; this produces C3b deposition on the surface of opsonized cells, and once sufficient C3b accumulates, terminal complement proteins C6–C9 assemble into membrane attack complexes that mediate cell lysis. Despite this capacity, ocrelizumab is characterized by three- to fivefold lower CDC activity relative to rituximab, establishing its ADCC-skewed effector profile.

Antibody-dependent cellular phagocytosis provides a further depletion route: complement activation generates opsonins C3b and C4b on the target surface, which engage complement receptors on macrophages and other effector cells to drive phagocytosis. In parallel, FcγR-bearing myeloid cells mediate trogocytosis — the removal and internalization of both antibody and CD20 from the B-cell surface — which can modulate target density and alter susceptibility to subsequent effector mechanisms.

Direct cell death represents a minor additional pathway for type I antibodies such as ocrelizumab. Across the anti-CD20 class, type II antibodies such as obinutuzumab and tositumomab induce programmed cell death through a caspase-independent pathway, but a contribution from direct apoptotic signaling via CD20 cross-linking has also been proposed for ocrelizumab, particularly in lymphoid tissue compartments.

In terms of kinetics, CD19+ B cells are almost completely depleted by week 2 after the first 600 mg dose and remain extensively depleted across 24-week dosing intervals. After treatment stops, slow repopulation begins approximately 6 months after the last infusion, with median repletion taking over 15 months. Depletion is more efficient in peripheral blood than in lymphoid organs; B-cell depletion in lymphoid organs and the CNS is limited, which helps explain the relative safety margin of the therapy. Beyond canonical B cells, ocrelizumab also depletes CD20-dim T-cell subsets in both CD4 and CD8 compartments — cells characterized by a proinflammatory phenotype with high production of IFN-γ, TNF-α, and GM-CSF.

These cellular events translate to a downstream immunological shift: removal of B cells reduces antigen presentation and pathogenic T-cell activation, diminishes pro-inflammatory cytokine production, and moves the immune environment toward immune downregulation. At the signaling level, ocrelizumab treatment is associated with significant reductions in PKCβII, HIF-1α, and VEGF protein content in PBMCs after 12 months, consistent with counteraction of an overactive PKCβII→HIF-1α→VEGF inflammatory cascade. Collectively, this chain — from CD20 binding through multi-arm B-cell depletion to immune downregulation — provides the mechanistic basis for ocrelizumab's clinical efficacy in reducing relapse rates and slowing disability progression in multiple sclerosis.