Normal immune response
The normal immune response may be considered as being divided into three lines of defence:
Innate immune response
The innate immune response is the second line of defence against pathogens and is a non-specific immune response encompassing:
Adaptive immune response
The adaptive immune response is the third line of defence against pathogens.
General features:
Antigen presentation:
Cell-mediated immunity:
Humoral immunity:
- Physical and chemical barriers
- Innate immune response
- Adaptive immune response
Physical and chemical barriers prevent pathogen entry. If breached, the innate immune system provides a rapid, non-specific defence through phagocytes, NK cells, mast cells, cytokines, and complement. The adaptive immune system is slower but highly specific, relying on B and T lymphocytes, and generates long-lasting immunological memory.
Physical and chemical barriers
Physical and chemical barriers are the bodies first line of defence against pathogens seeking entry into the body. These barriers include:- Skin: Provides a tough, waterproof barrier preventing pathogen entry.
- Mucous membranes: Line the respiratory, gastrointestinal, and genitourinary tracts; trap microbes in mucus.
- Cilia: In the respiratory tract, move mucus (and trapped pathogens) upwards for clearance.
- Secretions: Acid in the stomach, lysozyme in tears/saliva, and antimicrobial peptides disrupt microbes.
- Normal flora: Compete with pathogens for space and nutrients.
Innate immune response
The innate immune response is the second line of defence against pathogens and is a non-specific immune response encompassing:
- Phagocytes: Include granulocytes (neutrophils, eosinophils, basophils) and monocytes/macrophages; ingest microbes and debris.
- Natural Killer (NK) cells: Detect infected or cancerous cells lacking normal “self” markers and kill them.
- Mast cells: Release histamine and other mediators, initiating inflammation.
- Inflammatory response (see box below)
Inflammatory response:
- Vasodilation: heat and redness
- Increased vascular permeability: oedema
- Stimulation of nociceptors:§ pain
- Cytokines (e.g. IL-8) which recruit neutrophils (chemotaxis)
- IL-1 and other mediators may induce fever
- Pattern Recognition Receptors (PRRs): Detect pathogen-associated molecular patterns (PAMPs), allowing rapid, non-specific defence.
- Complement system: A plasma protein cascade that opsonises pathogens, lyses microbes (MAC formation), recruits phagocytes, and promotes inflammation.
Adaptive immune response
The adaptive immune response is the third line of defence against pathogens.
General features:
- Antigen-specific but slower to develop (~5 days, peaking at 1–2 weeks).
- Generates immunological memory so there are faster, stronger responses on re-exposure.
Antigen presentation:
- MHC I presents intracellular antigens to CD8⁺ cytotoxic T cells.
- MHC II presents extracellular protein antigens to CD4⁺ helper T cells (on APCs: dendritic cells, macrophages, activated B cells).
Cell-mediated immunity:
- Cytotoxic T cells (CD8⁺) proliferate after antigen recognition and destroy infected cells.
- Helper T cells (CD4⁺) release cytokines to activate B cells, macrophages, and cytotoxic T cells.
Humoral immunity:
- B cells recognise antigens and, with T helper cell support, undergo clonal expansion.
- Differentiate into plasma cells secreting antibodies.
- Antibodies neutralise toxins/viruses, opsonise pathogens, activate complement, and mediate antibody-dependent cellular cytotoxicity (ADCC).
- Memory B cells persist long-term, enabling rapid secondary responses.