The Human Immune System

Understanding the Human Immune System

Physical Defences against Pathogens

• The first line of defence against pathogens is the skin and the mucous membranes lining the respiratory, digestive, and reproductive tracts.
• Skin serves as a physical barrier, and it secretes oils that contain antibacterial substances.
• Mucous membranes secrete mucus that trap pathogens, while cilia move the mucus and the captured pathogens out of the body.

The Innate and Adaptive Responses

• The human immune system has two major components, the innate response and the adaptive response.
• The innate response is the body’s initial, generic defence against pathogens, which includes mechanisms like inflammation, phagocytosis, and fever.
• Phagocytosis involves the ingestion and destruction of pathogens by white blood cells called phagocytes.
• The adaptive response is a slower, more specific reaction to particular pathogens. It includes reactions involving B cells and T cells.

B Cells, T Cells, and Antibodies

• B cells produce antibodies in response to pathogen invasion.
• Antibodies are proteins that bind to specific antigens on pathogens, either neutralising them or marking them for destruction by other immune cells.
• T cells are involved in cell-mediated immunity, with some types of T cells releasing substances that directly kill infected cells.
• Other T cells help to stimulate B cells to produce antibodies.

Antigens and Memory Cells

• Antigens are molecules present on the pathogens, which the immune system recognises and reacts against.
• When the body is exposed to an antigen for the first time, it forms memory cells that remember the specific antigen.
• Memory cells enable a faster and more efficient immune response if the same antigen is encountered again in the future.

Immunisation and Herd Immunity

• Immunisation or vaccination introduces a safe form of the pathogen (either dead or weakened) to the body to stimulate memory cell production.
• Immunisation promotes herd immunity, where a significant proportion of the population is immune, reducing the likelihood of disease outbreak.

The Role of the Lymphatic System

• The lymphatic system helps in defending the body against pathogens by transporting and exposing pathogens to lymphocytes.
• Lymphocytes are a type of white blood cell that actively participate in immune responses.
• Organs like the lymph nodes and the spleen have a high concentration of lymphocytes and are the sites where most immune responses are triggered.