What is Immunity? - Definition & Types

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  • 0:01 What is Immunity?
  • 1:06 Innate Immunity
  • 2:51 Adaptive Immunity
  • 5:05 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

This lesson will introduce immunity and define what immunity is and what it does. The lesson will also identify the different types of immunity and what they involve.

What is Immunity?

If you could have a superpower, what would you choose? Super speed or strength? Invisibility? Have you ever considered immunity?

Immunity? How is that a superpower? Immunity means being protected from something and being unaffected or not bothered by it. Let's say you have immunity to heat - this means heat can't bother you whatsoever - walking up to hot molten lava (about 2,000 degrees F!) would be no different than walking up to a river.

Amazingly, we already have a form of this superpower because of our immune system. Our immune system protects us and helps fight off disease. Microorganisms, small microscopic organisms, and viruses are everywhere. Ever thought about how many are on that door you just opened? Many microbes and viruses can cause disease and are termed pathogens.

With all these pathogens around, why aren't we sick every single moment? The answer is our immune system gives us different types of immunity to protect us from disease. There are two main types of immunity: innate, also called natural or inherited, and adaptive.

Innate Immunity

Plants and animals have what is called innate immunity. Innate immunity is the first line of defense against pathogens. It involves several cell types, proteins, and even an organ. The organ involved is your skin. Yes, skin is part of the first line of defense. It protects you and prevents pathogens from getting inside your body.

So, what are some ways a pathogen gets inside? Air, food, or a break in the skin are some ways a pathogen enters. A pathogen entering through food or air has mucus to go through. The mucosal surfaces prevent pathogens from attaching to cells and causing disease. A set of proteins called the complement system is also involved. The complement system attacks the pathogen and marks it for destruction.

A pathogen getting through skin and mucus will have to deal with several types of cells including phagocytes, eating cells, and natural killer (NK) cells before it can cause disease. Pathogens have warning flags on their surface that say: 'I don't belong here'.

Neutrophils, macrophages, and dendritic cells are all phagocytes. They recognize the warning flag, attack the pathogen, and eat it - a process known as phagocytosis. If a pathogen is too big for one cell alone, several cells attack at once.

NK cells on the other hand, identify infected cells (host cells) and activate the host cell's death receptor pathway or give the cell a lethal injection (injecting enzymes that degrade proteins). Host cells even try to fight back by turning off machinery that would help the pathogen and sending out distress signals.

If pathogens make it through all this, it's time for adaptive immunity to step in, and they do this with the help of dendritic cells.

Adaptive Immunity

Adaptive immunity works slower than innate, and is more specific. There are two types: passive and active. Passive immunity occurs when antibodies are passed from one person to another, as through transfusion for example.

The active immunity involves two types of white blood cells - T-cells and B-cells. Dendritic cells, after they have eaten and digested the pathogen, present the pathogen pieces to T-cells, which activates (turns on) the T-cells.

Cell-Mediated Immunity

T-cells are formed in the thymus and cruise around until activated. Since T-cells require direct contact with other cells, T-cell immunity is termed cell-mediated immunity. Activated T-cells become helper cells (TH) and cytotoxic (killer) T-cells. They recognize and cause the destruction of infected cells.

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