What Is the Immune System?

The immune system is the body's network of cells, tissues, organs, and molecules that protects against pathogens and other harmful biological threats.

Pathogens include:

• bacteria
• viruses
• fungi
• protozoa
• other disease-causing organisms or infectious agents

If a pathogen gets past the body's first-line defenses, such as the skin, mucus, and stomach acid, internal immune defenses can respond.

These defenses involve:

white blood cells.

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White Blood Cells

White blood cells, also called leukocytes, are cells involved in protecting the body against infection and other threats.

Unlike red blood cells, white blood cells have a:

nucleus.

They are produced mainly from stem cells in the:

bone marrow.

White blood cells can circulate through the blood and lymphatic system and can also move into body tissues.

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Different Types of White Blood Cells

White blood cells are not all the same.

Important groups include:

• neutrophils
• monocytes and macrophages
• lymphocytes
• eosinophils
• basophils

Different types perform different functions.

For this topic, three especially important groups are:

phagocytes, B lymphocytes, and T lymphocytes.

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Two Major Types of Immunity

The body's internal defenses can be divided broadly into:

innate immunity

and

adaptive immunity.

These systems are different, but they work closely:

together.

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Innate Immunity

Innate immunity is the body's rapid, general defense against pathogens.

It is present from birth and does not require previous exposure to a particular pathogen.

Innate defenses include:

• skin and other barriers
• mucus
• inflammation
• phagocytic cells
• natural killer cells
• antimicrobial proteins
• fever and other general responses

Innate immunity is generally:

rapid but non-specific.

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Adaptive Immunity

Adaptive immunity is a more specific defense system.

It recognizes particular:

antigens.

Important cells involved include:

• B lymphocytes
• T lymphocytes

Adaptive immunity can also produce:

immunological memory.

This allows the body to respond more rapidly if it encounters the same pathogen again.

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Comparing Innate and Adaptive Immunity

Innate immunity

Response:

rapid

Recognition:

general patterns associated with threats

Memory:

does not produce the highly specific long-term memory characteristic of adaptive immunity

Examples:

• phagocytes
• inflammation
• physical barriers

Adaptive immunity

Initial response:

usually slower

Recognition:

highly specific

Memory:

yes

Examples:

• B lymphocytes
• antibodies
• T lymphocytes

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What Is an Antigen?

An antigen is a molecule or molecular structure that can be specifically recognized by components of the adaptive immune system.

Antigens may be found on:

• bacteria
• viruses
• fungi
• infected cells
• other foreign materials

Different pathogens can carry different:

antigens.

These molecular differences allow the adaptive immune system to distinguish among different targets.

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What Happens When a Pathogen Enters?

Imagine that bacteria enter through a cut in the skin.

A simplified response might be:

pathogen enters → innate response begins → inflammation develops → phagocytes attack → pathogen antigens are recognized → adaptive immunity activates → specific lymphocytes multiply → antibodies and/or T-cell responses develop → memory cells remain

Several of these processes can occur at the same time.

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Phagocytes

A phagocyte is a cell capable of surrounding and engulfing particles such as microorganisms.

Important phagocytic cells include:

• neutrophils
• macrophages

The process of engulfing material is called:

phagocytosis.

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Phagocytosis

A simplified sequence of phagocytosis is:

1. Recognition

The phagocyte recognizes the microorganism or foreign material.

2. Engulfment

The cell membrane surrounds it.

3. Internalization

The pathogen becomes enclosed inside the cell.

4. Destruction

Cellular compartments containing digestive enzymes and antimicrobial substances help destroy the engulfed material.

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Neutrophils

Neutrophils are abundant white blood cells and important early responders during many bacterial infections.

They can:

• move toward sites of infection
• engulf microorganisms
• destroy pathogens
• release antimicrobial substances

Neutrophils are an important component of:

innate immunity.

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Macrophages

Macrophages are large phagocytic cells found in many body tissues.

They can:

• engulf pathogens
• remove damaged cells
• release signaling molecules
• help activate adaptive immunity

Macrophages therefore provide an important connection between:

innate and adaptive immunity.

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Inflammation

When tissue is damaged or infected, an:

inflammatory response

may develop.

Chemical signals cause changes in nearby blood vessels and attract immune cells.

Common signs include:

• redness
• heat
• swelling
• pain

Inflammation helps bring immune cells and molecules to the site of infection.

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Why Does Inflammation Cause Redness?

During inflammation, local blood vessels may:

dilate.

This increases blood flow to the area.

Greater blood flow contributes to:

redness and warmth.

Blood vessels also become more permeable, allowing fluid and immune components to move into the tissues.

This contributes to:

swelling.

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Lymphocytes

Lymphocytes are white blood cells particularly important in adaptive immunity.

Major types include:

B lymphocytes

and

T lymphocytes.

These cells recognize specific molecular targets.

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B Lymphocytes

B lymphocytes, or B cells, play a major role in antibody-mediated immunity.

When an appropriate B cell is activated, it can multiply and differentiate into:

plasma cells

and:

memory B cells.

Plasma cells produce large quantities of:

antibodies.

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What Is an Antibody?

An antibody is a protein produced by activated B cells, particularly plasma cells.

Antibodies bind specifically to particular:

antigens.

Antibodies are also called:

immunoglobulins.

They circulate in blood and other body fluids.

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Antibody Structure

A typical antibody has a roughly:

Y-shaped structure.

The tips contain regions that bind to particular antigens.

These are called:

antigen-binding sites.

The precise shape and chemical properties of these sites determine which antigen the antibody can recognize.

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Antibody Specificity

Antibodies are:

specific.

An antibody that binds strongly to one antigen will not necessarily bind to another.

A simplified model is:

specific antibody + matching antigen → binding

This specificity allows the adaptive immune system to target particular pathogens.

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What Do Antibodies Do?

Antibodies can help defend the body in several ways.

They may:

• block viruses from entering cells
• neutralize toxins
• cause pathogens or particles to clump together
• mark pathogens for destruction
• activate other immune mechanisms

Antibodies do not usually destroy pathogens simply by:

touching them.

Instead, binding can interfere with the pathogen or make it easier for other immune mechanisms to eliminate it.

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Neutralization

Some antibodies bind to parts of a virus or bacterial toxin.

This can prevent the pathogen or toxin from interacting normally with:

host cells.

This process is called:

neutralization.

For example:

antibody binds viral surface structure → virus has difficulty attaching to host cell

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Opsonization

Antibodies can coat the surface of a pathogen.

This can make it easier for phagocytes to recognize and engulf the pathogen.

This process is called:

opsonization.

Therefore:

antibody binds pathogen → phagocyte recognizes coated pathogen → phagocytosis becomes easier

This demonstrates cooperation between adaptive and innate immunity.

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Agglutination

Because antibodies can bind antigens, they may cause particles or cells carrying those antigens to:

clump together.

This is called:

agglutination.

Clumping can make foreign material easier for the immune system to remove.

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T Lymphocytes

T lymphocytes, or T cells, are another major component of adaptive immunity.

Important groups include:

• helper T cells
• cytotoxic T cells
• memory T cells

T cells have different roles from B cells.

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Helper T Cells

Helper T cells coordinate parts of the immune response.

They release signaling molecules that can help activate:

• B cells
• cytotoxic T cells
• macrophages
• other immune cells

They therefore act partly as:

coordinators.

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Cytotoxic T Cells

Cytotoxic T cells can recognize certain infected or abnormal body cells.

They can trigger these target cells to:

die.

This is particularly important during many:

viral infections,

because viruses reproduce inside host cells.

Destroying an infected cell can interrupt viral replication.

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B Cells vs T Cells

A useful distinction is:

B cells → antibodies

T cells → coordinate immune responses or attack infected cells

Both are important components of:

adaptive immunity.

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Clonal Selection

The body contains many different lymphocytes with different antigen receptors.

When a pathogen enters, only certain lymphocytes recognize its:

antigens.

Those matching cells become activated.

They then reproduce to produce many similar cells.

This is called:

clonal selection and expansion.

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Why Clonal Expansion Matters

Initially, there may be relatively few lymphocytes capable of recognizing a particular antigen.

After activation:

one matching lymphocyte → many matching lymphocytes

The immune system therefore builds a much larger population capable of responding to that specific:

threat.

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The Primary Immune Response

The first time the body encounters a particular antigen, it produces a:

primary immune response.

This response takes time to develop.

The general sequence is:

antigen encountered → specific lymphocytes activated → clonal expansion → effector cells produced → pathogen controlled → memory cells remain

During this period, the person may experience symptoms of disease.

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Memory Cells

After an infection is controlled, most activated immune cells eventually disappear.

However, some remain as:

memory cells.

These may include:

• memory B cells
• memory T cells

Memory cells can persist for long periods, although the duration varies depending on the infection.

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The Secondary Immune Response

If the same antigen is encountered again, memory cells can respond.

The secondary response is often:

faster and stronger

than the primary response.

A simplified comparison is:

First exposure → slower primary response → memory cells form

Second exposure → rapid activation of memory cells → stronger response

The pathogen may be controlled before noticeable disease develops.

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How Immunity Develops After Infection

Suppose a person becomes infected with a virus.

During infection:

1. Viral antigens are recognized.

2. Specific B and T cells become activated.

3. These lymphocytes multiply.

4. Effector cells help control the infection.

5. Some activated cells become memory cells.

After recovery, these memory cells may remain.

If the same pathogen is encountered again:

memory cells can produce a faster adaptive response.

This is one way immunity can develop after infection.

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Does Previous Infection Always Produce Complete Immunity?

No.

The strength and duration of immunity depend on factors such as:

• the pathogen
• the individual's immune response
• how much the pathogen changes
• time since infection

For some diseases, immunity may be long-lasting.

For others, reinfection can occur.

Therefore:

previous infection does not always mean permanent protection.

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Natural Active Immunity

When a person becomes infected and their own immune system develops memory, this is called:

natural active immunity.

It is:

natural

because exposure occurred through infection.

It is:

active

because the person's own immune system produced the response.

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Artificial Active Immunity

Vaccination can also stimulate immune memory.

This is called:

artificial active immunity.

A vaccine exposes the immune system to an antigen, or instructions for producing an antigen, without requiring the person to experience the full natural disease.

The immune system then develops:

memory.

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Vaccination and Immune Memory

A simplified vaccine response is:

vaccine antigen → lymphocyte activation → clonal expansion → antibodies and cellular responses → memory cells

Later:

pathogen exposure → memory cells activate rapidly → faster immune response

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Active vs Passive Immunity

Active immunity develops when a person's own immune system responds to an antigen.

This can occur through:

• infection
• vaccination

Passive immunity occurs when a person receives antibodies that were produced elsewhere.

Passive immunity can provide rapid protection but usually does not produce the same long-lasting immune memory.

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Natural Passive Immunity

Antibodies can pass from a parent to a baby.

For example, antibodies can cross the:

placenta.

Additional antibodies can be transferred through:

breast milk.

This provides temporary protection while the baby's immune system continues developing.

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Artificial Passive Immunity

Antibodies can sometimes be given medically.

These antibodies provide:

immediate protection.

However, because the recipient's immune system did not necessarily create the antibodies or memory cells:

protection is usually temporary.

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Comparing Active and Passive Immunity

Active immunity

Antibodies produced by:

the person's own immune system

Memory cells:

usually produced

Development:

takes time

Duration:

often longer-lasting

Examples:

infection and vaccination

Passive immunity

Antibodies obtained from:

another source

Memory cells:

not produced by the transferred antibodies

Development:

immediate

Duration:

temporary

Examples:

maternal antibodies and administered antibodies

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Innate and Adaptive Immunity Work Together

Innate and adaptive immunity should not be thought of as two completely separate:

systems.

They communicate continuously.

For example:

macrophage encounters pathogen

↓

phagocytosis occurs

↓

pathogen-derived antigens may be presented

↓

T cells become activated

↓

adaptive immune response develops

The innate system can therefore help initiate the adaptive response.

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Antigen Presentation

Some immune cells display fragments of pathogens on their surfaces.

This is called:

antigen presentation.

Important antigen-presenting cells include:

• dendritic cells
• macrophages
• B cells

Antigen presentation helps T cells recognize that a particular:

threat is present.

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Dendritic Cells

Dendritic cells are especially important for connecting innate and adaptive immunity.

They can capture material from pathogens and travel to lymphoid tissues.

There they present antigens to:

T lymphocytes.

This helps initiate a specific adaptive immune response.

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Lymph Nodes

Lymph nodes are small structures found throughout the lymphatic system.

They contain many:

immune cells.

Lymph nodes provide locations where immune cells can encounter antigens and communicate with one another.

During infection, nearby lymph nodes may become:

swollen.

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Why Do Lymph Nodes Swell?

During an immune response:

• lymphocytes multiply
• immune cells accumulate
• fluid may increase

This can cause lymph nodes to enlarge.

Swollen lymph nodes therefore often indicate that the immune system is:

responding to something.

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Fever and Immunity

During some infections, immune signaling can increase the body's temperature set point.

This produces:

fever.

Fever is part of the body's broader response to infection.

However, fever itself is not the same thing as:

immunity.

It is one component of the body's response.

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Worked Example 1

Bacteria enter through a cut.

A neutrophil surrounds and engulfs a bacterium.

What process has occurred?

Phagocytosis.

This is part of:

innate immunity.

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Worked Example 2

A B lymphocyte recognizes an antigen and becomes activated.

What may happen next?

The B cell undergoes:

clonal expansion.

Some cells differentiate into plasma cells that produce:

antibodies.

Others can become:

memory B cells.

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Worked Example 3

A virus enters a body cell and begins reproducing.

Which immune cell can directly target the infected cell?

A:

cytotoxic T cell.

The T cell can trigger the infected cell to die.

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Worked Example 4

A person encounters a pathogen for the second time.

Their immune system responds much more quickly than during the first infection.

Why?

Memory cells formed during the first immune response.

These cells allow a faster and often stronger:

secondary immune response.

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Worked Example 5

A newborn receives antibodies from its mother.

What type of immunity is this?

Natural passive immunity.

The antibodies provide protection, but the baby did not produce those transferred antibodies through its own adaptive response.

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Worked Example 6

A person receives a vaccine and later develops memory B and T cells.

What type of immunity has developed?

Artificial active immunity.

The person's own immune system responded to the vaccine antigens.

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Worked Example 7

An antibody binds to a virus and prevents the virus from attaching to a host cell.

What process is occurring?

Neutralization.

The antibody interferes with the virus's ability to infect the cell.

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Worked Example 8

An antibody coats a bacterium, making it easier for a macrophage to engulf.

This demonstrates:

adaptive and innate immunity working together.

The antibody is part of adaptive immunity.

The macrophage and phagocytosis are part of innate immunity.

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Comparing the Three Layers of Defense

First line

Main purpose:

prevent pathogen entry

Examples:

• skin
• mucus
• cilia
• stomach acid

Innate internal defenses

Main purpose:

rapidly attack pathogens that enter

Examples:

• phagocytes
• inflammation
• natural killer cells

Adaptive immunity

Main purpose:

specifically target particular antigens and develop memory

Examples:

• B cells
• antibodies
• T cells
• memory cells

Together these form a powerful:

layered defense system.

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Common Mistake: All White Blood Cells Produce Antibodies

They do not.

Antibodies are produced by:

plasma cells derived from activated B lymphocytes.

Other white blood cells perform different functions.

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Common Mistake: Antibodies Eat Pathogens

Antibodies do not perform:

phagocytosis.

Phagocytes engulf pathogens.

Antibodies bind to specific antigens and can neutralize threats or help other immune cells recognize them.

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Common Mistake: Innate Immunity Is the Same as Adaptive Immunity

Innate immunity is generally:

rapid and non-specific.

Adaptive immunity is:

specific and capable of forming immunological memory.

Both are essential.

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Common Mistake: Antibodies Kill Every Pathogen Directly

Antibodies have several functions.

They may:

• neutralize
• mark pathogens
• activate other immune mechanisms
• cause agglutination

Other immune components often complete the destruction of the pathogen.

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Common Mistake: Immunity Means You Can Never Be Infected Again

Immunity is not always:

complete or permanent.

Protection can weaken.

Pathogens can change.

Different people may develop different levels of protection.

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Common Mistake: Infection Is the Only Way to Develop Immunity

Vaccination can also produce:

adaptive immune memory.

Vaccination allows immune memory to develop without requiring the full natural disease.

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Common Mistake: Passive Immunity Creates Memory Cells

Passive immunity provides:

ready-made antibodies.

Because the recipient's adaptive immune system did not necessarily generate the response, passive immunity generally does not create the same lasting memory as active immunity.

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Check Your Understanding

1. What are white blood cells?

2. Where are most white blood cells produced?

3. Give three examples of white blood cells involved in immunity.

4. Define innate immunity.

5. Define adaptive immunity.

6. Give two differences between innate and adaptive immunity.

7. What is an antigen?

8. What is a phagocyte?

9. Describe phagocytosis.

10. What role do neutrophils play in immunity?

11. What role do macrophages play?

12. What is inflammation?

13. Why does an infected area often become red and swollen?

14. What is a lymphocyte?

15. Name the two major types of lymphocytes.

16. What is the main role of B lymphocytes?

17. What is a plasma cell?

18. What is an antibody?

19. Explain why antibodies are specific.

20. Describe two ways antibodies can help protect the body.

21. What is neutralization?

22. What is opsonization?

23. What is the role of helper T cells?

24. What is the role of cytotoxic T cells?

25. Why are cytotoxic T cells particularly important during viral infections?

26. What is clonal selection?

27. What is clonal expansion?

28. Describe the primary immune response.

29. What are memory cells?

30. Explain the secondary immune response.

31. Why is the secondary response usually faster than the primary response?

32. Explain how immunity can develop after an infection.

33. Why does previous infection not always provide permanent immunity?

34. What is natural active immunity?

35. What is artificial active immunity?

36. Compare active and passive immunity.

37. Give an example of natural passive immunity.

38. Explain how vaccination produces immune memory.

39. Explain how innate and adaptive immunity work together.

40. A new pathogen enters the body through damaged skin. Describe how the innate and adaptive immune systems could respond from the moment the pathogen enters until long-term immunity develops.

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Key Terms

• White blood cell: Blood cell involved in immune defense.
• Leukocyte: Another name for a white blood cell.
• Immune system: Network of cells, tissues, organs, and molecules that protects the body against biological threats.
• Innate immunity: Rapid, general defense present without previous exposure to a specific pathogen.
• Adaptive immunity: Specific immune response involving lymphocytes and immunological memory.
• Antigen: Molecular structure recognized specifically by components of the adaptive immune system.
• Phagocyte: Cell capable of engulfing microorganisms and other particles.
• Phagocytosis: Process in which a cell surrounds, engulfs, and digests material.
• Neutrophil: Phagocytic white blood cell important in rapid innate responses.
• Macrophage: Large phagocytic cell involved in innate defense and activation of adaptive immunity.
• Inflammation: Local immune response to infection or tissue damage.
• Lymphocyte: White blood cell involved particularly in adaptive immunity.
• B lymphocyte: Lymphocyte responsible for antibody-mediated immune responses.
• Plasma cell: Activated B-cell descendant specialized for producing antibodies.
• T lymphocyte: Lymphocyte involved in coordinating immunity or targeting infected cells.
• Helper T cell: T cell that helps coordinate immune responses.
• Cytotoxic T cell: T cell capable of killing certain infected or abnormal cells.
• Antibody: Specific antigen-binding protein produced by plasma cells.
• Neutralization: Blocking the harmful activity of a pathogen or toxin through antibody binding.
• Opsonization: Coating a target so that phagocytes can recognize and engulf it more efficiently.
• Clonal selection: Activation of lymphocytes capable of recognizing a particular antigen.
• Clonal expansion: Multiplication of activated lymphocytes.
• Memory cell: Long-lived lymphocyte capable of responding rapidly to a previously encountered antigen.
• Primary immune response: Adaptive response following first exposure to an antigen.
• Secondary immune response: Faster adaptive response following later exposure to the same antigen.
• Active immunity: Immunity produced by a person's own immune response.
• Passive immunity: Temporary protection provided by antibodies obtained from another source.
• Antigen presentation: Display of antigen fragments to immune cells such as T lymphocytes.

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Key Takeaways

• White blood cells are essential components of the body's immune system.
• Different white blood cells perform different defensive roles.
• Innate immunity provides rapid, broad protection.
• Adaptive immunity targets specific antigens and can produce immunological memory.
• Neutrophils and macrophages can destroy microorganisms through phagocytosis.
• Inflammation helps recruit immune defenses to infected or damaged tissues.
• B and T lymphocytes are central to adaptive immunity.
• Activated B cells can develop into plasma cells.
• Plasma cells produce antibodies.
• Antibodies bind specifically to particular antigens.
• Antibodies can neutralize pathogens and toxins and help other immune mechanisms eliminate pathogens.
• Helper T cells coordinate immune responses.
• Cytotoxic T cells can destroy infected cells.
• Clonal selection activates lymphocytes that recognize a particular antigen.
• Clonal expansion produces many cells capable of responding to that antigen.
• The first encounter with an antigen produces a primary immune response.
• Some activated lymphocytes remain as memory cells.
• Memory cells allow a faster and often stronger secondary immune response.
• Immunity can develop naturally after infection.
• Vaccination can also produce adaptive immune memory.
• Active immunity involves the person's own immune response and usually produces memory.
• Passive immunity provides ready-made antibodies and is usually temporary.
• Previous infection does not necessarily provide permanent or complete immunity.
• Innate and adaptive immunity cooperate rather than acting independently.
• Physical barriers, innate defenses, and adaptive immunity provide multiple layers of protection against infectious disease.