Innate Immune Response

The innate immune system is the body’s first line of defense against infection. It provides a rapid, nonspecific response to invading pathogens such as bacteria, viruses, fungi, and parasites — often within minutes to hours.

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🧩 Key Characteristics

• Present from birth (inborn immunity)

• Nonspecific — responds the same way to most invaders

• No memory — does not improve upon repeated exposure (unlike adaptive immunity)

• Rapid activation — acts immediately or within hours of infection

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🛡️ Main Components

1. Physical and Chemical Barriers

• Skin – physical barrier; secretes antimicrobial peptides.

• Mucous membranes – trap microbes in respiratory and gastrointestinal tracts.

• Secretions – such as stomach acid, lysozyme in tears/saliva, and defensins.

2. Cellular Defenses

• Phagocytes:

  • Neutrophils – first responders that engulf and destroy microbes.

  • Macrophages – long-lived cells that engulf pathogens and dead cells.

• Natural Killer (NK) cells: destroy virus-infected or cancerous cells.
• Dendritic cells: capture antigens and link the innate and adaptive immune systems.

3. Molecular and Chemical Mediators

• Cytokines – signaling proteins (e.g., TNF-α, IL-1) that coordinate inflammation.

• Complement system – plasma proteins that opsonize (tag) microbes and cause cell lysis.

• Acute-phase proteins – like C-reactive protein (CRP) that enhance pathogen clearance.

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🔥 Major Innate Immune Responses

• Inflammation: Redness, heat, swelling, and pain due to increased blood flow and immune cell recruitment.

• Fever: Elevated body temperature inhibits microbial growth.

• Phagocytosis: Engulfing and digesting pathogens.

• Interferon response: Limits viral replication.

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🔄 Connection to Adaptive Immunity

While innate immunity acts first, it also activates the adaptive immune system by presenting antigens to lymphocytes (T and B cells), guiding a more specific and long-term immune response.

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Would you like me to show a diagram summarizing how innate and adaptive immunity interact?

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