There are five distinct classes of leukocytes (white blood cells) that coordinate to provide defence against infectious disease:
Neutrophils
- Most abundant type of white blood cell and the first responder to microbial infection
- They are unable to renew their lysosomes and die after having phagocytosed a few pathogens (forms the majority of pus)
- Analogy: Standard police officer – quick to respond to the situation, but lacks special training or skills and so dies rapidly
Eosinophils
- Prominent at the sites of allergic reactions and parasitic infections (rare in blood but common at mucous membranes)
- Do not phagocytose pathogens but instead release chemical products which perforate cell membranes
- Consequently, they function as the primary response to large multicellular parasites (e.g. helminth infections)
- Analogy: Fumigator – specialised to deal with pests / parasites (e.g. helminths) by releasing chemical products
Basophil
- Basophils are chiefly responsible for initiating inflammatory responses by releasing the chemicals histamine and heparin
- Functionally they are similar to mast cells, however they circulate in the bloodstream whereas mast cells are localised
- Because they promote inflammation, they are common contributors to allergic responses
- Analogy: Fireman – the leukocyte involved when a region is inflamed (‘in flames’)
Monocyte
- Monocytes are the largest type of leukocyte and share phagocytosis duties with neutrophils
- They are slower to respond than neutrophils but are longer lasting, as they can renew their lysosomes for continued digestion
- Monocytes will differentiate into two types of cells in response to pathogenic infection – macrophages and dendritic cells
- Macrophages will remain in the tissue and phagocytose, whereas dendritic cells present antigen fragments to lymphocytes
- Analogy: Riot police (macrophage) – slower to respond than standard police but better prepared and survives for longer
- Analogy: Signalman (dendritic cell) – identifies the pathogen and sends signals to the appropriate special forces (lymphocytes)
Lymphocyte
- Lymphocytes are responsible for the production of antibodies which target specific antigens present on pathogens
- They are more common in the lymphatic system than blood and are slowest to respond (requiring antigen presentation)
- Lymphocytes include B cells (which become antibody-secreting plasma cells) and T cells (which mediate B cell activity)
- Lymphocytes are also involved in the destruction of virus-infected body cells (via cytotoxic T cells and natural killer cells)
- Analogy: Special forces / superheroes – takes longest to mobilise but specially trained to target specific pathogens
Types of Leukocytes
⇒ Click on the diagram to show as an analogy
Prevalence of Leukocytes
The relative proportions of the different types of white blood cells are:
• Neutrophils (roughly 60 – 70%)
• Lymphocytes (roughly 20 – 30%)
• Monocytes (approximately 1 – 6%)
• Eosinophils (approximately 1 – 3%)
• Basophils (less than 1%)
Mnemonic: Never Let Monkeys Eat Bananas