Adaptive Immunity

1. What Is Adaptive Immunity?

Adaptive immunity is the specific, slower, but stronger arm of the immune system that:

• Targets specific antigens

• Develops memory

• Responds much faster and stronger upon re-exposure

Key Purposes:

• Protects against future infections

• Explains why vaccines work

2. Innate vs Adaptive Immunity

This section compares the two main branches of the immune system.

• Innate immunity fills the gap during the first 5–7 days

• Adaptive immunity takes over once antigen-specific responses are produced by B and T cells

3. Primary vs Secondary Immune Response

Describes how the immune system reacts to first and subsequent exposures to an antigen.

• Primary Exposure (first time):

  • Innate response acts first

  • Adaptive response is slower, not specific

  • Memory cells are formed

• Secondary Exposure (infection or vaccine):

  • Much faster (24–48 hours)

  • Much stronger

  • Both innate and adaptive responses activate

  • Memory B and T cells respond immediately

Note: Vaccination and natural infection produce the same immune memory.

4. Memory in Adaptive Immunity

Adaptive immunity remembers past infections.

• Memory cells:

  • Live for decades

  • Exist in higher numbers than naïve cells

  • Explain lifelong immunity (e.g., MMR vaccine)

Clonal Expansion:

1. One specific B or T cell recognizes an antigen

2. That cell proliferates into thousands of identical cells

3. Most die after infection clears

4. A small number become memory cells

5. Humoral vs Cellular Immunity

• Humoral Immunity

  • Mediated by B cells

  • Produces antibodies

  • Antibodies circulate in blood and lymph

• Cellular Immunity

  • Mediated by T cells

  • Targets infected or abnormal cells

6. Antibody Specificity & Epitopes

• Each antibody recognizes one specific epitope

• An epitope = a 3D structure on an antigen

• A single protein can have multiple epitopes

Explains:

• Why the immune system is so effective

• Genetic drift in viruses

7. Antibody Functions

Major Antibody Actions:

1. Opsonization: Tags pathogens for phagocytosis

2. Neutralization: Blocks binding of toxins/viruses to cells

3. Agglutination: Clumps pathogens together, making them easier to eliminate

Note: Antibodies do not kill directly—they tag or block.

8. B Cell Differentiation

After activation, B cells become:

• Plasma cells

  • Antibody factories

  • Short-lived (14–21 days)

• Memory B cells

  • Long-lived

  • Enable rapid secondary response

9. T Cell Diversity & Tolerance

• T Cell Receptor Diversity:

  • Generated by genetic recombination

  • Each person has a unique immune repertoire

  • Explains differences in immunity between individuals

10. T Cell Selection (Tolerance)

• Positive Selection (Thymus):

  • Ensures T cells can be activated

  • Nonfunctional T cells are eliminated

• Negative Selection:

  • Eliminates T cells that recognize self antigens

  • Prevents autoimmunity

  • Failure of negative selection = autoimmune disease

11. Types of Immunity

• Active Immunity (long-term):

  • Immune system does the work

  • Generates memory

• Passive Immunity (short-term):

  • Antibodies are given

  • No immune memory formed

12. T Cell Subsets (High-Yield)

• Cytotoxic T Cells (CD8+):

  • Directly kill infected or cancer cells

  • Target intracellular pathogens

  • Require antigen presented on MHC I

• Helper T Cells (CD4+):

  • Do not kill

  • Release cytokines to:

    • Activate B cells

    • Activate macrophages

    • Recruit immune cells

  • Essential for a functional immune response

  • HIV targets CD4+ T cells → immune collapse

13. Big Picture Summary

• Adaptive immunity = specificity + memory

• B cells → antibodies

• T cells → kill or coordinate

• Vaccines safely create immune memory

• Secondary responses are faster and stronger