The Immune System Overview

Definition and Purpose

The immune system is the body’s defense mechanism that protects against pathogens- disease-causing microorganisms such as:

• Bacteria

• Viruses

• Protozoa

• Parasites

• Fungi

It prevents infection, illness, and tissue damage through highly coordinated defense mechanisms.

1. Two Major Types of Immunity

Type	Speed	Specificity	Memory	Examples
Innate (nonspecific)	Fast	Nonspecific (attacks anything foreign)	No memory	Skin, mucous membranes, phagocytes, antimicrobial proteins
Adaptive (specific)	Slower initial response	Highly specific (targets antigens)	Has memory	B cells, T cells, antibodies

Key Difference:

• Innate = first and second lines of defense

• Adaptive = third line of defense

2. INNATE IMMUNITY (Nonspecific Defense)

First Line of Defense

Physical and chemical barriers that prevent entry:

• Skin

• Mucous membranes

• Stomach acid

• Tears, saliva, mucus

• Antimicrobial proteins

Second Line of Defense

Internal defenses that act after a pathogen enters:

• Phagocytes (neutrophils, macrophages)

• Natural killer (NK) cells

• Fever and inflammation

• Antimicrobial proteins

  • Complement system

  • Interferons

Complement Cascade

• A group of ~20 proteins that circulate in the blood.

• Activate upon detecting foreign invaders.

• Three main functions:

Function	Description
1. Membrane Attack Complex (MAC)	Proteins form pores in the pathogen membrane → cell lysis (death).
2. Inflammation	Releases inflammatory molecules → attracts immune cells to the site.
3. Opsonization	“Tags” pathogens for destruction by macrophages (“eat me” signal).

Interferons

• Released by virus-infected cells.

• Signal neighboring cells to produce antiviral proteins for self-protection.

• Acts as a “warning system” to nearby cells:

  “I’m infected — make defenses before the virus gets you.”

A self-sacrificial defense that limits viral spread.

3. White Blood Cells (Leukocytes)

Mnemonic:  “Never Let Monkeys Eat Bananas”
(From most to least abundant)

Cell Type	Function	System Type
Neutrophils	First responders; phagocytose bacteria; nonspecific “eaters.”	Innate
Lymphocytes	B & T cells; adaptive immune response; memory formation.	Adaptive
Monocytes	Clean-up cells (“molecular mops”); become macrophages in tissue.	Innate
Eosinophils	Fight parasites; involved in allergic responses.	Innate
Basophils	Release histamine; cause inflammation (“burning”).	Innate

Leukocyte = White Blood Cell
All WBCs = Leukocytes, but only lymphocytes belong to the adaptive system.

4. ADAPTIVE IMMUNITY (Specific Defense)

Activated when the innate system cannot clear infection.
Provides specific targeting and long-term immunity.

Two Main Lymphocyte Types:

Lymphocyte	Primary Function
T cells	Directly attack infected or abnormal cells.
B cells	Produce antibodies that tag invaders for destruction.

T CELLS — Cell-Mediated Immunity

Two types of T cells:

T Cell Type	Marker	Function
Helper T cells (Th)	CD4	Activate and direct other immune cells; stimulate B cells to make antibodies.
Cytotoxic T cells (Tc)	CD8	Destroy virus-infected or cancerous cells.

5. MHC (Major Histocompatibility Complex)

Acts like “flag holders” on cell surfaces to display antigens (foreign markers).

MHC Type	Where Found	Antigen Source	Interacts With	Effect
MHC Class I	On all body cells	From inside infected cells (viruses, tumors)	Cytotoxic T cells (CD8)	Cell destruction (self-sacrifice)
MHC Class II	On professional antigen-presenting cells (APCs) — e.g., macrophages, dendritic cells, B cells	From engulfed pathogens	Helper T cells (CD4)	Stimulates immune response & antibody production

MHC I Example (Cytotoxic T Cell Activation)

1. A cell is infected by a virus.

2. Viral antigen fragments are placed on MHC I and displayed on the surface.

3. CD8 (Cytotoxic T cell) recognizes and binds.

4. Infected cell is destroyed to stop viral spread.

(Self-sacrifice for the body’s protection.)

MHC II Example (Helper T Cell Activation)

1. A professional APC (e.g., macrophage) engulfs a pathogen.

2. It presents pathogen fragments (antigens) on MHC II.

3. CD4 (Helper T cell) binds to this MHC II-antigen complex.

4. The T helper cell proliferates and stimulates B cells → antibody production.

5. Leads to specific and amplified immune response.

Summary Comparison

Feature	MHC I	MHC II
Displayed by	All nucleated cells	Professional APCs
Presents	Endogenous (internal) antigens	Exogenous (external) antigens
Recognized by	CD8 Cytotoxic T cells	CD4 Helper T cells
Result	Infected cell killed	Activation of immune response and antibody production

Adaptive Memory

• After infection, memory B cells and T cells remain in circulation.

• On re-exposure, they respond faster and stronger — often preventing illness entirely.

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• Quick Review Summary

Concept	Key Idea
Innate Immunity	Fast, nonspecific, no memory (skin, phagocytes, complement, interferons).
Adaptive Immunity	Slow initially, specific, has memory (B & T cells).
Complement Cascade	Proteins that lyse pathogens, promote inflammation, and tag invaders.
Interferons	Viral alarm molecules that warn neighboring cells.
MHC I	Found on all cells; signals cytotoxic T cells to destroy infected cells.
MHC II	Found on APCs; activates helper T cells to coordinate immune response.
“Never Let Monkeys Eat Bananas”	Mnemonic for WBCs: Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils.