General Principles of Viral Defense
For decades, medical practice has relied on two effective methods for combating viral infections, encompassing both prevention and treatment: active and passive immunization.
Both approaches share a global goal—preventing viruses from realizing their pathogenic potential in the human body. However, the pathways to this goal, the mechanisms of the agents used, and the clinical scenarios for their selection differ fundamentally. The choice between active and passive immunization depends on whether the body needs to be prepared for a potential future encounter with an infection or requires immediate protection here and now following exposure.
Active Immunization: How Vaccines Work
Active immunization (vaccination) involves the administration of specific antigens into the body. The immune system recognizes these antigens and initiates a cascade of reactions resulting in the production of endogenous antibodies.
From a pharmacodynamic perspective, the primary target for these synthesized antibodies is specific viral envelope proteins. The mechanism of action involves antibodies binding to these proteins and physically blocking adsorption—the attachment of the virus to the host cell membrane. By preventing viral entry into the cell, the infectious process is halted.
The primary clinical goal of vaccines is routine prophylaxis: reliably preventing infection before exposure. Infections controlled in this manner include measles, rubella, mumps, and hepatitis B.
A notable exception to the general rule is the rabies vaccine. This is a unique example of the post-exposure therapeutic use of active immunization. The vaccine is administered to the patient not before, but directly after infection (e.g., following an animal bite). The goal is to induce immunity rapidly enough to prevent disease development.
Passive Immunization: Ready-Made Defense
Passive immunization differs fundamentally from active immunization in that preformed antibodies are administered directly to the patient. The body does not spend time synthesizing them, providing immediate protection. The primary tools here are immunoglobulin preparations.
The most widely used agent is normal human immunoglobulin, composed predominantly of immunoglobulin G (IgG). Its mechanism of action prevents viral adsorption onto the surface of susceptible cells. An important pharmacodynamic feature of normal immunoglobulin is that beyond specific viral neutralization, it exhibits a pronounced nonspecific immunostimulatory effect.
This preparation is administered exclusively via the intramuscular route. Indications for its use are broad:
- Emergency prophylaxis: Used when exposure has already occurred; effective against measles, hepatitis A, pertussis, influenza, and poliomyelitis.
- Prevention of various bacterial and viral infections.
- Inclusion in combination therapy for established infections.
- Administration in immunosuppressed patients whose own immune systems cannot mount an adequate response.
Monoclonal Antibody Preparations
A modern advancement in passive immunization is the use of monoclonal antibodies. Unlike normal human immunoglobulin, which contains a polyclonal pool of diverse antibodies, monoclonal preparations exhibit absolute specificity for a single specific epitope.
A prominent example is palivizumab. Its specificity is strictly targeted solely against the respiratory syncytial virus (RSV).
In clinical practice, palivizumab has a clearly defined indication: it is used for the prevention of RSV infections in pediatric patients at high risk for severe disease.