Second line of defence
The second line of defence provides a non-specific response to pathogens that have breached the body’s initial barriers. Unlike the first line of defence, which aims to prevent pathogens from entering, the second line involves various cellular and molecular mechanisms that target and eliminate these invaders once inside the body.
Inflammatory Response
The inflammatory response acts as a rapid, non-specific reaction to injury or infection. Its primary purpose is to eliminate the initial cause of cell injury, clear out damaged cells and tissues, and establish a repair process. Key players in this response include prostaglandins, histamine, neutrophils, macrophages and natural killer cells.
Prostaglandins
Prostaglandins are lipid compounds that promote vasodilation, which is the widening of blood vessels. Vasodilation increases blood flow to the affected area, making it easier for immune cells to reach the site of infection or injury. This process also increases the permeability of blood vessel walls, allowing immune cells, nutrients, and other healing factors to move into the tissues. Prostaglandins also contribute to the pain and fever associated with inflammation, acting as signals that alert the body to tissue damage.
Histamine
Histamine is a chemical released by mast cells and basophils in response to tissue injury or infection. It plays a role in inflammation by promoting vasodilation, increasing the permeability of blood vessels and allowing immune cells to access the affected area more easily. This leads to the characteristic redness, swelling and heat associated with inflammation. Histamine also contributes to the attraction of other immune cells to the site of infection, enhancing the overall immune response.
Neutrophils
Neutrophils are the first immune cells to arrive at the site of infection or injury. They are a type of phagocyte that engulfs and digests pathogens through a process called phagocytosis. Neutrophils release enzymes and reactive oxygen species that help destroy microbes, but they are short-lived and often die after a single round of phagocytosis, forming pus in the process.
Macrophages
Macrophages are larger phagocytic cells that arrive after neutrophils and are capable of engulfing and digesting larger pathogens, dead cells, and debris. Unlike neutrophils, macrophages can survive longer and continue to function throughout the inflammatory response. They also play a role in signalling other immune cells to the area, releasing cytokines that further amplify the response. Macrophages are crucial for the clean-up and repair phase, removing dead cells and pathogens and promoting tissue healing.
Natural Killer (NK) Cells
Natural killer (NK) cells are a type of lymphocyte that plays a unique role in the inflammatory response by targeting and destroying infected or abnormal cells, such as virus-infected cells or cancerous cells. NK cells recognise cells that lack normal self-markers and induce apoptosis, a programmed cell death, in these compromised cells. They release perforins and granzymes that puncture the cell membrane and trigger cell death pathways. NK cells help control infections by preventing the spread of infected cells, thereby supporting other immune responses and reducing the overall burden of pathogens.
Complement
The complement system consists of a series of proteins that circulate in the blood and bodily fluids in an inactive state. Upon encountering pathogens, these proteins are sequentially activated in a cascading manner, amplifying the immune response.
Once activated, the complement system performs several key functions:
Opsonisation
One of the primary roles of the complement system is to enhance the process of opsonisation, where complement proteins coat the surface of pathogens. This coating acts as a beacon for phagocytic cells, such as neutrophils and macrophages, improving their ability to recognise and engulf the microbes. This increases the efficiency of phagocytosis.
Chemotaxis and inflammation promotion
Complement proteins act as chemical signals that attract immune cells to the site of infection, a process known as chemotaxis. By recruiting and activating phagocytes and other immune cells, the complement system enhances the overall inflammatory response, helping to contain and eliminate the infection.
Membrane Attack Complex (MAC) Formation
The membrane attack complex is a structure that inserts itself into the membrane of a pathogen. This complex forms pores in the cell membrane, leading to cell lysis (bursting) and death of the pathogen. This direct killing mechanism is particularly effective against bacteria and certain viruses.
Macrophages
Macrophages are larger phagocytic cells that arrive after neutrophils and are capable of engulfing and digesting larger pathogens, dead cells, and debris. Unlike neutrophils, macrophages can survive longer and continue to function throughout the inflammatory response. They also play a role in signalling other immune cells to the area, releasing cytokines that further amplify the response. Macrophages are crucial for the clean-up and repair phase, removing dead cells and pathogens and promoting tissue healing.
Natural Killer (NK) Cells
Natural killer (NK) cells are a type of lymphocyte that plays a unique role in the inflammatory response by targeting and destroying infected or abnormal cells, such as virus-infected cells or cancerous cells. NK cells recognise cells that lack normal self-markers and induce apoptosis, a programmed cell death, in these compromised cells. They release perforins and granzymes that puncture the cell membrane and trigger cell death pathways. NK cells help control infections by preventing the spread of infected cells, thereby supporting other immune responses and reducing the overall burden of pathogens.