Second line of defence (component of the innate immune system, providing a non-specific and immediate protect against potential pathogens that have entered the body cellular and non-cellular components)

Cellular components of the second line of defence (all cells involved are called leukocytes (white blood cells))

• phagocytes

• natural killer cells

• mast cells

• eosinophils

Phagocytosis

a process in which phagocytes consume and destroy foreign or dead material present in the body by engulfing it through endocytosis

once engulfed, lysosomes containing lysozymes (antimicrobial enzymes) destroy the foreign or dead material by fusing with the vesicles containing the engulfed material

Phagocytes

cells that engage in phagocytosis

• to communicate within the immune system, phagocytes release cytokines

  
  

Cytokines

• important cell signalling molecules that help protect against pathogens, and can help guide immune cells to the site of injury/infection, allowing them to function optimally

Types of Phagocytes

• neutrophils (initial responders, short lasting)

• macrophages (longer lasting, can present antigens)

• dendritic cells (longer lasting, can present antigens)

macrophages and dendritic cells are antigen-presenting cells (APCs)

• after undergoing phagocytosis, they present antigens from the consumed material on their surface

• APCs are specific immune cells, which therefore express MHC Class 2 markers and use them to present the consumed antigens on their surface to then interact with the adaptive immune systems

Natural Killer Cells

large granulated cells that target both abnormal and virally infected cells with insufficient MHC Class 1 markers

this is achieved with the presence of killer inhibitory and killer activation receptors;

• killer inhibitory receptor

examines the surface of cells for MHC Class 1 markers

• killer activation receptors

binds to certain molecules which appear on cells undergoing cellular stress

if a killer activation receptor is activated and the killer inhibitory receptor is unable to bind to a sufficient number of MHC Class 1 markers, cell death is initiated

if the killer inhibitory receptor detects a sufficient number of MHC Class 1 markers, it overrides the killer activation signal, preventing cell death

• however, factors (eg. viral infection, cancer) can destroy or suppress the production of MHC Class 1 marker

Mast Cells

causes inflammation through the release of histamine

• reside in connective tissues throughout the body

• when they detect injury to surrounding cells or are stimulated by antigens or allergens, they become activated and degranulate, releasing histamine (important in the inflammatory response)

Eosinophils

releases toxic chemical mediators to destroy invading pathogens 

• typically target pathogens that are too large to be phagocytosed, by degranulating on contact with them and releasing the chemical mediators contained within their granules 

Non-Cellular components of the second line of defence

• interferons

• complement proteins

• cytokines

Interferons

released by virally-infected cells which causes changes to neighbouring cells that make them less susceptible to infection

when a cell is infected with a virus, it releases interferons (a type of cytokine)

interferons interact with receptors on neighbouring cells, causing them to undergo a number of changes that make them less susceptible to viral infection

• this helps prevent the virus from spreading between cells

Complement proteins

react with each other and aid in the destruction of pathogens via opsonisation, attraction of phagocytes to pathogens, and formation of membrane attack complexes

within the blood, there are many different complement proteins that together form the complement system

in the presence of certain pathogens, these proteins begin reacting with each other in a series of reactions called the complement cascade