Covers the two arms of immunity, innate and acquired, starting with external and internal defence mechanisms and the three barriers of innate immunity. It then goes through phagocytes, the complement system with its alternative pathway and membrane attack complex, inflammation, and extracellular killing by eosinophils, NK cells and macrophages. The note ends with adaptive immunity, B and T lymphocytes, and the structure of antibodies, including heavy and light chains and the Fab region.
Topic 19 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 2 sections, about 1,065 words (7 min of reading), with 25 flashcards.
In this note
Introduction
Humoral Immunity: Structure of Antibodies
Introduction
Immunity provides the defense of organism against bacteria, viruses, fungi etc. Two main kinds of immunity exist: nonspecific immunity and specific one.
The nonspecific immunity is innate; it is directed to many antigens.
The specific immunity is acquired; it is directed to certain antigen.
Two mechanisms underlie the nonspecific immunity :
External defense mechanisms – e.g. the acidity of the gastric juice, the entity of the mucous membranes etc.
Internal defense mechanisms – they include opsonization, phagocytosis, fever, interferon production, natural killer (NK)cells action.
3 barriers of innate immunity:
Physical barrier, skin.
Chemical barrier, Low pH, fatty acids, secretions etc.
Biological barrier, competition by normal skin bacteria and other micro-organisms
Normally pathogens won’t be able to penetrate skin or cross mucosal membranes. But if it does happen, due to injury or the pathogen has the capability to enter. There are 2 defences:
- Humoral response: this is based on free molecules in solution that can react to a pathogen. e.g. antibodies secreted by B-cells.
- Cellular response: is based on direct cell interaction with a pathogen. e.g. phagocytes.
Phagocytosis is “engulfing” of the foreign particles by the phagocytic cells, followed by lysis of the particles.
The main phagocytic cells are: Ne, Mo, macrophages (derived from Mo), some organ-specific phagocytes: microglia in the brain (also derived from Mo), Kupffer cells in liver, phagocytic cells in spleen and lymph node.
The phagocytic cells are usually attracted by the carbohydrats of the bacterial wall (chemotaxis).
Phagocytes, has two cell types:
- Neutrophils, is small and has segmented nucleus (polymorphonuclear), short lived 1-2 days. Found only in bloodstream.
- Monocyte, is large and has one big nucleus, it is long lived. Found in different tissues. After they settle they become macrophages.
The complement is a group of plasma proteins (C1 – C9), which mediate phagocytosis,control inflammation and interact with antibodies.
Complement System Overview:
The complement system consists of inactive proteases circulating as precursors.
When triggered, these pro-proteins activate other pro-proteins in a cascade, forming the complement cascade.
Opsonization:
Opsonization involves covering pathogens with complement proteins (mainly C3b).
This process marks the pathogen for phagocytosis by immune cells.
Membrane Attack Complex (MAC):
The MAC creates a hole in the pathogen’s membrane, causing it to swell and burst.
Components of the MAC include complement proteins C5b, C6, C7, C8, and C9.
Alternative Complement Pathway:
Triggered by polysaccharides on bacteria and fungi.
Key steps:
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