Question
Classify immunity. Add a note on passive immunity.
Answer
Classification of immunity:
- Innate (natural/non-specific) immunity — present from birth, non-specific, rapid, no memory.
- Acquired (adaptive/specific) immunity:
- Active immunity — the individual’s own immune system generates the response.
- Natural active — following actual infection.
- Artificial active — following vaccination.
- Passive immunity — pre-formed antibody is transferred from an external source.
- Natural passive — maternal antibody across the placenta/via colostrum.
- Artificial passive — immunoglobulin/antitoxin injection.
- Active immunity — the individual’s own immune system generates the response.
Note on passive immunity Passive immunity confers resistance to infection by transferring pre-formed antibody into an individual, without stimulating that individual’s own immune system to produce the antibody.
Characteristics: acts immediately (no lag period, unlike active immunity, which requires clonal expansion), but protection is short-lived, since the transferred antibody is progressively catabolised and no antigen-specific memory cells are generated.
Types:
- Natural passive immunity — transplacental transfer of maternal IgG (the only immunoglobulin class crossing the placenta), and transfer of secretory IgA via colostrum/breast milk.
- Artificial passive immunity — deliberate administration of pre-formed antibody preparations, e.g., Human Tetanus Immunoglobulin (HTIG), diphtheria antitoxin, rabies immunoglobulin, and pooled human immunoglobulin.
Clinical use: valuable when immediate protection is required (e.g., after a high-risk exposure — animal bite for rabies, contaminated wound for tetanus) and there is insufficient time for active immunization to develop protective immunity, or when the individual cannot mount an adequate active response (e.g., immunodeficiency).