Introduction
Clinical immunology studies the immune system in health and disease. This topic covers immunity as a biological phenomenon, its types (innate and adaptive, humoral and cellular, active and passive), and the basic humoral and cellular mechanisms behind protective and immunopathological reactions: hypersensitivity (allergy), autoimmunity and immunodeficiency.
Immunity as a biological phenomenon
- Immunity is the state of an organism having the biological defenses to fight a disease.
- It is defined as the ability of multicellular organisms to resist harmful microorganisms from entering it.
- Immunity is a complex biological system.
- A key function is that the immune system recognises and tolerates self (own body).
- It involves both specific and non-specific components.
Types of immunity
Natural resistance (innate immunity)
- Also referred to as the non-specific response; it is quick to respond and present/active from birth.
- It is the non-specific, immediate response to pathogens. It lacks immunologic memory, is short-lasting, and does not improve upon subsequent exposures.
- It consists of physical, chemical, molecular, and cellular defences.
- Major components include skin and mucous membranes, phagocytic leukocytes (neutrophils and macrophages), specialised lymphocytes (NK cells), and several plasma proteins including proteins of the complement system.
- It can distinguish self from non-self. The response is rapid and prevents the establishment of infection.
- Specific mechanisms:
- physical barriers;
- chemical barriers (lysozymes, pH, lipids, enzymes, complement factors, interferons, acute phase proteins, antimicrobial peptides);
- normal human flora (microbiota);
- cellular factors such as phagocytosis, inflammatory response, and fever.
Acquired immunity (adaptive immunity)
- The immune system that is pathogen specific and not genetically conferred.
- It is based on antibodies and specifically reactive T lymphocytes, acquired through immune system stimulation by microbial antigens.
- It develops after a latent period of 1-2 weeks.
- Immune memory develops, leading to stronger responses in subsequent infections: the secondary response is accelerated and augmented. It provides long-lasting protection.
- Adaptive immunity has two types: humoral and cell mediated.
- Humoral immunity is immunity mediated by macromolecules in extracellular fluids (humours), including antibodies and complement proteins. It involves the production of antibodies by plasma cells derived from activated B-cells. It protects the body by neutralizing pathogens, facilitating opsonization, activating complement, and preventing pathogen adherence. Antibodies can only attack viruses outside of their host cells.