BIO 5.3-5.6 - 1st and 2nd line of defence

Two major subdivisions of immunity

1. Innate, non-specific immunity

2. Adaptive, specific immunity

Innate immunity

The type of immunity that is present from birth, producing non-specific immune responses against pathogens

Adaptive immunity

An immune response that is specific to a particular antigen and develops through contact with an antigen

First line of defence

Consists of physical, chemical, and microbiological barriers to prevent pathogens from entering the body

Examples of physical barriers in animals

• Intact skin - the dead outer layer of skin prevents pathogens from entering the body

• Mucous membranes - Pathogens are trapped in mucus secreted by goblet cells and carried by wafting cilia towards the glottis where they are swallowed

• Coughing and sneezing - expels foreign particles from the breathing tract

Examples of chemical barriers in animals

• Lysozyme in tears, saliva and nasal secretions splits bacterial cell walls

• Stomach acid has a low pH that kills bacteria

Microbiological barriers in animals

• Non-pathogenic microbes (normal flora) lives on surfaces of the body, preventing the growth of other bacteria

  • This is achieved by competing for space and resources and reducing the pH of the microenvironment

Examples of physical barriers in plants

• The waxy cuticle and outer epidermal cells prevents the entry of pathogens like skin in mammals

• Bark provides a physical barrier like the cuticle

• Thorns, spines and prickles deter animals that could carry vector-based pathogens

Examples of chemical barriers in plants

• Saponins are proteins stored in vacuoles that can damage the cell membranes of pathogens when released

• Defensins are proteins which block the growth of fungal cells

Second line of defence

Consists of the actions of immune cells and proteins in a non-specific, innate response against pathogens that have gained entry to the body

Mechanisms of the second line of defence (3)

• Phagocytosis

• Defensive proteins

• Inflammatory response

Phagocytes

A type of white blood cell that can engulf and destroy foreign material

Phagocytosis (steps)

The bulk movement of solid material into a cell

1. Receptors on the phagocyte bind to foreign pathogens

2. The phagocyte engulfs the pathogen in a vesicle called a phagosome

3. Lysosomes fuse with the phagosome

4. The lysosomes release digestive chemicals that destroy the pathogen

5. Indigestible material is exocytosed from the cell

4 types of phagocytes

1. Neutrophils

2. Monocytes

3. Macrophages

4. Dendritic cells

Neutrophils

• Irregular many-lobed nuclei and granular cytoplasm

• Engulf and digest bacteria

• Release defensins and hydrogen peroxide that disrupt bacterial and fungal membranes

• Release cytokines that attract other immune cells and cause inflammation

Monocytes

• The largest of the phagocytes manufactured in the bone marrow

• Differentiate into macrophages

Macrophages

• Larger than neutrophils, with regular, horseshoe-shaped nuclei and non-granular cytoplasm

• Complement proteins made by macrophages recognise pathogens as non-self

• Macrophages are antigen presenting cells and release cytokines

Dendritic cells

• Have surface projections that resemble the shape of dendrites

• Abundant in interfaces between the external and internal environments of the body (skin, lungs, gut)

• Actively motile

• Antigen presenting cells

• Can migrate to lymph nodes and activate T cells

Eosinophils

Contain granules with toxic chemicals and histamine which are released when in contact with large parasitic agents that cannot be engulfed

Natural killer cells

• Cytotoxic cells that provide non-specific protection against viruses and cancer

• Perforin punches holes in cell membranes and triggers apoptosis of abnormal cells

• Release cytokines to attract and activate immune cells

Mast cells

• Mediate the inflammatory response

• Contains granules with cytokines, histamine and heparin which increase the inflammatory response

Defensive molecules (4)

• Complement proteins

• Cytokines

• Interferons

• Chemokines

Complement proteins

• Always present in the blood and made by macrophages and monocytes

• Become active when in contact with antigen-antibody complexes

• Contributes to inflammation by increasing permeability of capillaries and attracting phagocytes

• Destroy bacterial membranes

• Opsonising pathogens (signalling for phagocytes to engulf and digest them)

Cytokines

• Small proteins, glycoproteins or peptides that indicate the presence of damage or pathogens and send signals to immune cells

• Can promote growth of lymphocytes, induce fever, promote antibody responses, activate macrophages and initiate inflammation

Interferon

• A type of cytokine that is produced by and act on cells infected by viruses

• Interfere with the virus replication inside cells by triggering the production of enzymes that break down viral RNA and proteins

• Attract natural killer cells that kill virus-infected cells

Chemokine

• A type of cytokine that acts as a chemical attractant

• Attracts leukocytes to the sites of infection and inflammation

Steps of the inflammatory response

1. Patho