Innate (Nonspecific) Defenses

Two kinds of immunity. The immune system has two arms:

• Innate (nonspecific) immunity: present from birth, it responds immediately and the same way to any pathogen. It has no "memory."
• Adaptive (specific) immunity: develops over time, targets specific pathogens, and remembers them (covered next).

Innate immunity is the front line and the focus here. It has two "lines of defense."

First line — surface barriers. These physically and chemically block pathogens from entering:

• Skin: the intact, keratinized surface is a tough barrier (recall the integumentary system); breaks in it invite infection.
• Mucous membranes: line entryways (respiratory, digestive, urinary tracts) and trap pathogens in mucus, often swept away by cilia.
• Chemical defenses: stomach acid, enzymes in saliva and tears (lysozyme), and the skin's acidic surface kill or discourage microbes.

If pathogens breach these barriers, the second line responds.

Second line — internal innate defenses. These act inside the body against invaders that get past the surface:

• Phagocytes: cells like neutrophils and macrophages engulf and destroy pathogens (recall these white cells).
• Natural killer (NK) cells: attack and destroy infected or abnormal (including some cancer) cells.
• Inflammation: a coordinated response to injury or infection (below).
• Antimicrobial proteins: including interferons (which help protect cells from viruses) and the complement system (proteins that punch holes in pathogens and boost other defenses).
• Fever: a raised body temperature that slows some pathogens and speeds immune responses.

Inflammation. Inflammation is the innate response to tissue damage or infection, producing the classic signs: redness, heat, swelling, and pain (and sometimes loss of function). It occurs because injured tissue releases chemicals (like histamine) that dilate blood vessels (bringing more blood — redness and heat) and make them leaky (letting fluid and immune cells into the tissue — swelling). This delivers phagocytes and clotting factors to the site, contains the damage, and begins repair (recall wound healing). Inflammation is protective, though excessive or chronic inflammation can be harmful.

Fever. Fever is a systemic innate response: pyrogens (fever-causing substances) reset the hypothalamic thermostat to a higher point, so the body warms up. Moderate fever can inhibit pathogen growth and enhance immune activity — an example of the hypothalamus and homeostasis (set-point change) at work.

How it works

Layered innate defense:

First line: skin + mucous membranes + chemicals (block entry)
   ↓ (if breached)
Second line: phagocytes + NK cells + inflammation + antimicrobial proteins (complement, interferons) + fever
   ↓ (if not enough)
Adaptive immunity takes over (next sections)

Comparisons

Common confusions

• Innate vs adaptive. Innate = fast, general, no memory; adaptive = slower, specific, with memory.
• Inflammation is protective, not just a problem — it's a normal, helpful response (though chronic inflammation harms).
• Fever is a controlled response (set-point change), not just "the body overheating."
• Complement is part of innate immunity (proteins), though it also assists adaptive responses.

Memory aids

• Inflammation signs: "Red, Hot, Swollen, Painful."
• First line = "barriers (skin/mucus)"; second line = "cells + chemicals + fever."
• NK cells = "Natural Killers of infected/abnormal cells."

Quick review

• Innate immunity is fast, general, and has no memory; adaptive immunity is specific and remembers.
• First line: skin and mucous membranes plus chemical defenses block entry.
• Second line: phagocytes, NK cells, inflammation, antimicrobial proteins (complement, interferons), and fever.
• Inflammation (redness, heat, swelling, pain) recruits defenses and starts repair; fever is a controlled set-point rise that aids defense.