Innate immune response: First and second lines of defence - TCE Biology (Tasmania)
“Immune response: vertebrates have innate (non-specific) and adaptive (specific) immune responses in three lines of defence; the first line is innate, non-specific physical and chemical barriers; the second line is innate, non-specific cellular, chemical and inflammatory responses involving phagocytes; cell specialisation (phagocytic cells) and the role of the cell membrane in endocytosis, including phagocytosis”
Vertebrates have three lines of defence. The first two are innate (non-specific): they respond the same way to any pathogen. The first line is barriers that stop pathogens entering: physical (skin, mucous membranes, cilia), chemical (stomach acid, enzymes in tears and saliva) and biological (the normal bacteria of the skin and gut). The second line acts once a pathogen gets in: the inflammatory response, in which mast cells release histamine, blood flow increases and phagocytes (neutrophils, macrophages and dendritic cells) engulf and digest pathogens by phagocytosis. The third line is adaptive (specific).
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What this dot point is asking
The innate response is part of criterion 7 (Section D of the TASC exam). The external assessment specifications include first-line barriers (chemical, structural and biological), the inflammatory response and the cells involved (mast cells, neutrophils, macrophages, dendritic cells), phagocytosis, and macrophages and dendritic cells as antigen-presenting cells.
Three lines of defence
| Line | Type | What it does |
|---|---|---|
| First | Innate, non-specific | Barriers that stop pathogens entering the body |
| Second | Innate, non-specific | Cellular, chemical and inflammatory responses to pathogens that get in |
| Third | Adaptive, specific | Responses targeted at particular antigens, with memory (see adaptive immune response) |
The first line: barriers
- Physical (structural): intact skin with a tough outer layer of dead cells; mucous membranes lining the airways and gut; mucus that traps pathogens; cilia that sweep mucus out of the airways; hairs in the nose.
- Chemical: acid in the stomach; lysozyme (an enzyme that breaks down bacterial cell walls) in tears and saliva; slightly acidic sweat and oils on the skin.
- Biological: the normal microbiota (harmless bacteria on the skin and in the gut) compete with pathogens for space and nutrients.
The second line: inflammation and phagocytosis
The inflammatory response
- Damaged cells and mast cells release chemicals, including histamine.
- Histamine causes local blood vessels to dilate, increasing blood flow: the area becomes red and warm.
- Capillaries become more permeable, so fluid, and the cells and proteins it carries, leak into the tissue: the area swells, and pressure on nerves causes pain.
- Chemical signals attract phagocytes from the blood to the site.
Phagocytic cells
- Neutrophils: the most common white blood cells; first to arrive; engulf pathogens and die, forming pus.
- Macrophages: large, long-lived phagocytes that engulf pathogens and debris, and act as antigen-presenting cells.
- Dendritic cells: phagocytes in tissues that contact the environment (such as skin and airways); they engulf pathogens and carry antigens to lymph nodes to present them to T cells.
Phagocytosis and the cell membrane
Phagocytosis is a form of endocytosis, in which the cell membrane surrounds material and brings it into the cell.
- The phagocyte recognises the pathogen as non-self by molecules on its surface.
- The cell membrane extends around the pathogen and engulfs it, enclosing it in a vesicle (phagosome).
- A lysosome fuses with the vesicle and its digestive enzymes break the pathogen down.
- Macrophages and dendritic cells then display fragments of the pathogen (antigens) on their surface as antigen-presenting cells, which links the innate response to the adaptive response.
Innate responses do not target a particular antigen and do not produce memory. They respond quickly, within minutes to hours, while the adaptive response takes days to build up.
A cut on the knee
- First line breached: the cut breaks the skin, letting bacteria in.
- Second line: mast cells release histamine; the area becomes red, warm and swollen as blood flow and capillary permeability increase. Neutrophils arrive and engulf bacteria by phagocytosis, followed by macrophages.
- Link to the third line: dendritic cells and macrophages present bacterial antigens to T cells, starting the adaptive response if the infection persists.
Marker's note: name the specific cell and chemical at each step.
- Calling all white blood cells phagocytes
- Lymphocytes (B and T cells) are not phagocytes; they belong to the adaptive response.
- Saying mast cells engulf pathogens
- Mast cells release histamine to start inflammation; neutrophils, macrophages and dendritic cells engulf pathogens.
- Leaving out the membrane
- The course links phagocytosis to the cell membrane's role in endocytosis, so describe the membrane engulfing the pathogen.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original5 marksA splinter carries bacteria into the skin of a student's finger, and the area becomes red, warm and swollen. Explain how the inflammatory response produces these signs and helps to remove the bacteria.Show worked answer →
One mark for each step, up to five.
- Damaged cells and mast cells release chemicals, including histamine.
- Histamine causes blood vessels in the area to dilate, increasing blood flow, which makes the area red and warm.
- Histamine also makes capillaries more permeable, so fluid leaks into the tissue, causing swelling.
- Chemical signals attract phagocytes (first neutrophils, then macrophages) out of the blood into the tissue.
- The phagocytes engulf and digest the bacteria by phagocytosis, and macrophages clear away debris.
Original4 marksDescribe the steps of phagocytosis by a macrophage, referring to the role of the cell membrane.Show worked answer →
One mark for each step.
- The macrophage recognises the pathogen as foreign (non-self) by the molecules on its surface.
- The cell membrane extends around the pathogen (pseudopodia) and engulfs it by endocytosis, enclosing it in a vesicle (phagosome).
- The vesicle fuses with a lysosome, and digestive enzymes break down the pathogen.
- The macrophage can display pieces of the pathogen (antigens) on its surface, acting as an antigen-presenting cell to activate the adaptive response.