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How our immune system works? Innate and adaptive immunity.

 

​Understanding the Human Immune System: Our Internal Shield

​The human immune system is a complex network designed to protect us from harmful invaders like bacteria, viruses, and parasites. It operates through two primary systems: Innate Immunity and Adaptive Immunity.

Image by Pikisuperstar on Freepik

1. Innate Immunity: The First and Second Lines of Defense

​Innate immunity is non-specific, meaning it reacts the same way to every pathogen it encounters. It is composed of our first and second lines of defense.

​The First Line of Defense (Physical & Chemical Barriers)

​This is the body’s "border security," designed to prevent intruders from entering in the first place.

  • ​The Skin: The epidermis consists of epithelial cells that eventually die and shed off every 28 days. Sweat contains lysozyme, an enzyme that kills pathogens, while sebum (skin oil) keeps the skin moist and acidic to discourage bacterial growth.

  • ​Physical Traps: Tiny hairs in the nose and eyelashes trap dust and bacteria before they can enter the body.

  • ​Mucous Membranes: Aside from the skin, bacteria can enter through openings like the mouth, eyes, ears, and genitals. These openings are lined with mucous membranes containing Goblet cells, which secret mucus to trap bacteria.

  • ​Excretion: The body uses physical processes like urination, vomiting, and diarrhea to expel waste and pathogens quickly.

​The Second Line of Defense (Innate Immune Cells)

​If a pathogen breaches the first line, it confronts the second line of defense. This involves specialized White Blood Cells (WBCs) that provide a rapid, non-specific response.

  • ​Macrophages: These are large cells that engulf pathogens and fuse them with lysosomes to destroy them. They also release cytokines chemical signals that recruit other immune cells to the area.

  • ​Neutrophils: Often the first responders to an infection site. They circulate in the blood, quickly migrate to the site of an attack, and destroy invaders through phagocytosis.

  • ​Natural Killer (NK) Cells: These cells do not attack the pathogen directly; instead, they scan our own cells for signs of infection or cancer. If a cell is compromised, the NK cell triggers it to undergo "apoptosis" (programmed cell death).

  • ​Mast Cells: Found in connective tissues and mucous membranes, these cells release histamine to trigger inflammation, signaling other cells to help.

  • ​Basophils: These cells release histamine into the bloodstream to increase blood flow and assist in the inflammatory response.

  • ​Eosinophils: Specifically designed to deal with multicellular parasites (like worms) by attaching to them and releasing toxic proteins.

​2. Adaptive Immunity: The Third Line of Defense

​Adaptive immunity is the "Bridge" between the immediate response and long-term protection. It is highly specific and involves a sophisticated "memory" of past invaders.

​The Role of Dendritic Cells

​Dendritic cells act as the messengers. While other WBCs are fighting, the Dendritic cell captures fragments of the pathogen and represents them on its surface (antigen presentation). It then travels through the lymphatic vessels to a lymph node.

​T Cell and B Cell Activation

  1. ​Finding a Match: In the lymph node, the Dendritic cell finds a Helper T cell that matches the specific fragment of the pathogen.

  2. ​Cloning: Once activated, the Helper T cell begins to clone itself and divides into two groups:

    • ​Effector Cells: One group stays to fight the current pathogen.

    • ​Memory Cells: The other group goes into "storage" so the body can react faster if the same pathogen returns.

  3. ​B Cell Activation: The Helper T cell also activates B cells. Once activated, B cells transform into Plasma Cells, which release millions of antibodies specialized proteins that hunt down and neutralize the specific pathogen.

​Long-Term Memory

​After the battle is won, most of the active WBCs (like macrophages and plasma cells) undergo self-destruction as they are no longer needed. However, the Memory T cells and Memory B cells remain in the body for years, ensuring that if that specific bacteria or virus ever returns, the immune system can wipe it out before you even feel sick.


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