U4 AOS1 Topic 2: Second Line of Defense

When pathogen enter inside the body another defense system called as second line of defense comes into action. Second line of defense is a component of innate immune system which has non-specific response to pathogens. It includes different cells and molecules to kill the pathogens and foreign particles that have entered the body preventing the spread of pathogens

Cellular component of second line of defense:

The cells involved in this defense system include leukocytes also known as white blood cells. Leukocytes are of different types i.e. Phagocytes, natural killer cells, eosinophils, mast cells.

·        Phagocytes: These are the group of leukocytes which engulf the foreign materials, dead cells and pathogens by a process of endocytosis. Once engulfed, the vesicle containing pathogens is fused with lysosomes containing antimicrobial enzymes to digest these pathogens. Examples of phagocytes are neutrophils, macrophages and dendritic cells.

·        Natural killer cells: These are a class of leukocytes which detect and kill the infected host cells. These cells have two types of receptors i.e. killer inhibitory receptors that can detect and bind to the MHC class I markers on infected host cells and killer activation receptors that can bind to stress molecule present on host cell undergoing cellular stress.

·        Mast cell: These cells are present in connective tissues throughout the body and release histamine whenever they are stimulated by any antigen from surrounding cells. Histamine results in inflammatory response.

·        Eosinophils: These are the leukocyte cells which release certain toxic chemical mediators such as DNases, RNases and proteases whenever they encounter any pathogen.

Non-cellular components of second line of defense:

In addition to leukocytes, there are some non-cellular components that participate in second line of defense. These include complement proteins, interferon and fever initiation.

·        Complement proteins: There are several different complement proteins in blood. Once these proteins are activated, they bring out a complex sequence of reactions known as complement cascade which include:

1.    Opsonization where complement proteins bind to surface of pathogens.

2.    Chemotaxis where phagocytes are attracted towards pathogens.

3.    Lysis where a pore called as membrane attack complex is created in pathogen by complement proteins allowing the pathogen to burst and disintegrate.

·        Fever: It is a response of body towards pathogens where body temperature rises so that pathogens do not survive at this temperature.