Pathogens and innate (non-specific) immunity

Unit 5 · Animal Form and Function

Types of pathogens and how they spread, and the first and second lines of defence: barriers, phagocytes, inflammation, fever, interferons and complement.

Pathogens and disease

A pathogen is an organism that causes disease. Infectious diseases pass from one host to another.

Pathogen	Examples (including Sri Lankan diseases)
Viruses	Dengue (mosquito), influenza, COVID-19, hepatitis, HIV, rabies
Bacteria	Tuberculosis, cholera, typhoid, leptospirosis ("rat fever", spread by rat urine in paddy fields), tetanus
Fungi	Ringworm, athlete's foot, candidiasis
Protozoa	Malaria (Plasmodium, Anopheles mosquito), amoebic dysentery
Parasitic worms	Filariasis, hookworm, tapeworm

Transmission: by droplets and aerosols (influenza, TB), contaminated food and water (cholera, typhoid), direct contact and fomites, blood and body fluids (hepatitis B, HIV), vectors (mosquitoes: dengue, malaria and filariasis) and from mother to child. Sri Lanka was certified malaria-free by the WHO in 2016; dengue remains a major public-health problem. Control measures target the pathogen or its transmission: safe water, sanitation, vector control, vaccination and isolation.

First line of defence: barriers

These stop pathogens entering.

• Skin: the dead keratinised layer is a tough waterproof barrier; sebum and sweat make the surface acidic (about pH 5) and contain lysozyme; harmless skin microbes compete with pathogens.
• Mucous membranes of the nose and airways: sticky mucus traps microbes, cilia sweep them away; sneezing and coughing expel them.
• Chemical secretions: lysozyme (an enzyme that digests bacterial walls) in tears, saliva and nasal mucus; hydrochloric acid of the stomach (pH 2); acidic urine and vaginal secretions.
• Normal microbiota of the gut and skin compete with the pathogens for food and space.
• Mechanical flushing: tears, urine flow, diarrhoea and vomiting.
• Blood clotting seals wounds.

Second line of defence: the innate response

If the barriers are crossed, innate defences respond within minutes to hours. They recognise general features of pathogens (e.g. bacterial wall components) but not a specific species.

Phagocytes

Neutrophils (the most abundant white cells) and macrophages (made from monocytes) engulf pathogens by phagocytosis:

1. The phagocyte recognises the pathogen and binds to it.
2. It engulfs the pathogen by endocytosis into a phagosome.
3. Lysosomes fuse with it and the enzymes digest the pathogen.
4. Pieces (antigens) of the pathogen may be displayed on the cell surface (antigen-presenting cells) to activate the adaptive immunity.

Natural killer (NK) cells kill virus-infected cells and tumour cells by causing them to self-destruct (apoptosis).

Inflammation

When tissue is injured or infected, damaged cells and mast cells release histamine and other signals. The signals:

• dilate the arterioles and make the capillaries leaky, so that more blood (redness and heat) and more fluid (swelling) enter the area;
• attract phagocytes (chemotaxis) which squeeze out of the capillaries to reach the pathogens;
• cause pain and may activate clotting to wall off the infected area. Pus is a collection of dead white cells and debris.

The four signs of inflammation are redness, heat, swelling and pain.

Fever

Pyrogens raise the temperature set in the hypothalamus. A moderate fever (up to about 39 °C) inhibits the growth of many pathogens and speeds up the immune response.

Interferons and complement

• Interferons are proteins released by virus-infected cells. They bind to neighbouring cells and make them produce antiviral enzymes that block viral replication.
• Complement is a group of plasma proteins that are activated in a cascade by a pathogen or an antibody. They make holes in bacterial membranes, attract phagocytes and coat bacteria so they are easier to engulf (opsonisation).

Comparison of innate and adaptive immunity

Innate	Adaptive
Present from birth	Develops after exposure
Rapid (minutes to hours)	Slower (days) on first exposure
Same response to all pathogens	Specific to a particular antigen
No memory	Memory: faster, stronger second response
Phagocytes, NK cells, complement, barriers	B and T lymphocytes, antibodies

Exam tip: Be able to name which defences are first line (barriers) and which are second line (phagocytes, inflammation, fever, interferon, complement), and to link each sign of inflammation to the effect of histamine on the capillaries.