Antigen Localization and Core Mechanism
The pathogenesis of type II reactions always begins when antibodies locate their target on the cell surface. This antigen can have a dual origin. First, it may be a natural component of the plasma membrane (an intrinsic self-antigen). Second, it may be an exogenous (foreign) hapten or drug that has adsorbed onto the surface of an otherwise healthy cell.
The core of the pathologic reaction is that antibody binding to normal or altered cellular structures triggers one of several antibody-dependent mechanisms.
Complement-Dependent Reactions
In this mechanism, IgM or IgG antibodies play a primary role in close coordination with the complement system. Target cell damage occurs via two main pathways:
- Direct Lysis. Antibodies react with antigens on the cell surface, triggering complement activation. The cascade proceeds sequentially: after epitope binding to IgG and the C1q component, factors C1, C4, C2, C3, and subsequently C5b, C6, C7, C8, and C9 are activated. This results in the assembly of the membrane attack complex (MAC) on the membrane, which disrupts cellular integrity and leads to direct osmotic lysis and cell death.
- Opsonization. In this pathway, antibodies or the specific complement component C3b are fixed to the cell surface. The cell becomes "tagged" (opsonized), turning it into an attractive target for professional phagocytes (macrophages and neutrophils), which subsequently phagocytose the cell.
Clinical Examples of Complement-Dependent Damage
Several severe pathological conditions develop via the complement-dependent pathway described above:
- Transfusion Reactions. Occur during the transfusion of incompatible donor blood when recipient antibodies attack and destroy donor erythrocytes.
- Hemolytic Disease of the Newborn (Erythroblastosis Fetalis). Develops due to blood group incompatibilities between mother and fetus. Maternal IgG antibodies cross the placenta and cause massive destruction of fetal red blood cells.
- Autoimmune Cytopenias. Conditions where the body produces antibodies against its own blood cells, leading to their premature destruction. Examples include autoimmune hemolytic anemia, immune thrombocytopenia (ITP), and autoimmune neutropenia.
- Drug-Induced Reactions. Certain medications (e.g., penicillins) can adsorb to erythrocyte membranes, acting as haptens that trigger antibody-mediated destruction.
Antibody-Dependent Cellular Cytotoxicity (ADCC)
The defining feature of this mechanism is that it proceeds without complement fixation. The process begins when target cells are coated with low concentrations of IgG antibodies.
Effector cells—such as monocytes, neutrophils, eosinophils, or natural killer (NK) cells—then bind via their surface Fc receptors to the Fc region of the cell-bound IgG. This interaction triggers the release of perforins and granzymes from the effector cells, leading to direct target cell lysis without phagocytosis. Clinically, ADCC plays a critical role in acute and chronic graft rejection.
Antibody-Mediated Cellular Dysfunction
This is a unique variant of type II hypersensitivity in which antibodies bind to cell-surface receptors but do not cause direct cell lysis or acute tissue inflammation. Instead, they alter cellular physiology by either stimulating or blocking receptor function.
The classic clinical example of this state is Myasthenia Gravis. In this autoimmune disease, antibodies target acetylcholine receptors at the skeletal neuromuscular junction, blocking normal neuromuscular transmission and leading to profound muscle weakness.