Human Defences

This section explains human defences, covering, non-specific defences including phagocytes, specific defences such as white blood cells (lymphocytes). The human body is constantly exposed to harmful microorganisms such as bacteria, viruses, and fungi. To protect against these pathogens, the body has developed an intricate defence system. This system includes both non-specific defences, which act as general protection against a wide range of pathogens, and specific defences, which target pathogens more precisely.

Non-Specific Defences

Non-specific defences are the body's first line of defence against pathogens and are designed to protect the body in a general way, regardless of the type of pathogen. They do not target specific pathogens but provide immediate protection. Key non-specific defences include:

The Skin

The skin acts as a physical barrier, preventing pathogens from entering the body. It is tough, waterproof, and contains oils and acids that can inhibit the growth of many bacteria and fungi.

Mucous Membranes

The mucous membranes line the respiratory, digestive, and urogenital tracts, trapping pathogens with sticky mucus. The mucus can then be removed through coughing, sneezing, or swallowing.

Cilia in the Respiratory System

In the lungs and airways, tiny hair-like structures called cilia move mucus and trapped pathogens upwards towards the throat, where they can be coughed out or swallowed.

Stomach Acid

The stomach produces hydrochloric acid, which destroys many pathogens that enter through the mouth or nose with food or drink.

Tears and Saliva

Tears and saliva contain enzymes (like lysozyme) that break down the cell walls of some bacteria, helping to prevent infection.

The Inflammatory Response

When tissues are damaged or infected, the body responds with inflammation. This process includes increased blood flow to the affected area, which brings white blood cells to fight infection and promotes the healing of tissues.

Phagocytes (Non-Specific Defence)

Phagocytes are a type of white blood cell involved in the body's non-specific defence against pathogens. Their role is to ingest and destroy harmful microorganisms through a process called phagocytosis.

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Phagocytosis process:

1. The phagocyte identifies pathogens by recognising foreign markers (antigens) on their surfaces.
2. The phagocyte surrounds the pathogen with its cell membrane, forming a phagosome (a vacuole containing the pathogen).
3. The phagosome fuses with a lysosome (an organelle containing digestive enzymes).
4. The enzymes digest the pathogen, breaking it down into harmless components.
5. The phagocyte expels the waste products from the digestion.

Phagocytes are part of the innate immune system, meaning they are always present and ready to respond to a wide range of pathogens.

Specific Defences - White Blood Cells (Lymphocytes)

While non-specific defences provide immediate protection, the body also has specific defences to target particular pathogens. The key players in this specific immune response are lymphocytes, a type of white blood cell.

Lymphocytes and Antibodies

Lymphocytes play a crucial role in the adaptive immune system, which is more tailored to individual pathogens. There are two main types of lymphocytes involved in specific defences:

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B lymphocytes (B cells): These cells are responsible for producing antibodies, which are proteins that bind to specific antigens (molecules found on the surface of pathogens). When B cells encounter an antigen that matches their specific antibody, they produce many copies of that antibody to neutralise the pathogen.

• Antibody action: Antibodies can neutralise toxins, prevent pathogens from entering cells, and mark pathogens for destruction by phagocytes.

T lymphocytes (T cells): These cells help to coordinate the immune response. There are two main types of T cells:

• Helper T cells: These cells release chemicals that stimulate B cells to produce antibodies and activate phagocytes.
• Cytotoxic T cells: These cells directly attack and kill infected cells by recognising foreign antigens displayed on the surface of infected cells.

The process of immune recognition by lymphocytes is highly specific, as each lymphocyte can recognise only one particular pathogen.

Active Immunity

Active immunity occurs when the body’s immune system is exposed to a pathogen and produces a specific immune response. This includes the production of antibodies by B cells and the activation of T cells. Active immunity can occur in two ways:

Natural Active Immunity: This occurs when an individual is exposed to a pathogen, such as through infection. The body responds by producing antibodies and memory cells, which will recognise and fight off the pathogen if the body encounters it again in the future.

• Example: After having chickenpox, the body produces memory cells that provide immunity if the virus is encountered again.

Artificial Active Immunity: This occurs when the body is exposed to a weakened or inactivated form of a pathogen, often through vaccination. The vaccine stimulates the immune system to produce antibodies and memory cells without causing the disease itself.

• Example: Vaccination against measles, mumps, and rubella (MMR) provides immunity by stimulating the production of memory cells, which will respond rapidly if the person is later exposed to these viruses.

Summary

The human immune system has both non-specific defences (like the skin, mucous membranes, and phagocytes) that provide immediate, general protection against pathogens, and specific defences involving lymphocytes that target individual pathogens through the production of antibodies and the activation of T cells. Active immunity develops when the body is exposed to a pathogen or vaccine, leading to long-term protection through the production of memory cells. Together, these defences work to protect the body from a wide variety of diseases and infections.