Acquired Immunity

Acquired immunity (from Latin immunitas — immunity) is a highly specific form of the body’s immune defense. It is not innate.

This system develops over the course of a lifetime after direct contact with specific foreign antigens (as a result of a previous infection or vaccination) or through the administration of preformed antibodies to an individual.

Etiology and pathophysiology

Unlike innate immunity, which acts immediately but responds identically to all threats, acquired immunity takes days or weeks to become activated. Its function is mediated by two principal cell types:

• T lymphocytes — cellular immunity, which destroys infected cells;
• B lymphocytes — humoral immunity, which produces antibodies.

A fundamental property of acquired immunity is immunological memory. After the infection has been suppressed, most of the immune cells that prevailed die. However, a small fraction of them become memory T and B cells. They enter a “dormant state” and patrol the body for years.

If the same pathogen attempts to enter the body again, memory cells are immediately activated and mount a full-scale response even before the first symptoms of the disease appear.

Clinical significance

The concept of acquired immunity is the foundation of infectious disease medicine. It provides protection against reinfection with many diseases (chickenpox, measles, rubella).

In medicine, acquired immunity is divided into active and passive immunity:

• active immunity — is generated by the body itself after illness or vaccine administration. It takes longer to develop but provides protection for years;
• passive immunity — arises from the administration of preformed donor antibodies (for example, serum against tetanus or rabies) or from the transfer of maternal antibodies to the fetus through the placenta. Passive immunity acts immediately, saving lives in emergencies, but disappears after several months without leaving immunological memory.