Adaptive Immune System

Adaptive Immune System Factors

• Specific and Targeted

• Develops after an exposure to a pathogen

• Creates “memory” of it

What does the adaptive immune response do?

It relies on a small group of highly specialized cells that recognize specific antigens and remember them.

Antigen-Presenting Cells (APCs)

FUNCTION: Activate T cells by presenting antigens!

• Include:

  • Dendritic cells (most important for activation)

  • Macrophages

  • B cells (can also act as APCs)
    

• Engulf pathogens, process them, and present antigens on MHC II

• Bridge the innate and adaptive immune systems!

MHC - II

MHC II (Major Histocompatibility Complex class II) molecules are proteins found on the surface of certain immune cells. They are essential for the adaptive immune response.

• Found mainly on Antigen-Presenting Cells (APCs):

  • Dendritic cells

  • Macrophages

  • B cells

FUNCTION: Display pieces of pathogens (antigens) to helper T cells (CD4⁺)!

Antigen Presentation (MHC-II)

• APCs engulf pathogens via phagocytosis.

• The pathogen is broken into fragments (antigens).

• MHC II molecules carry these antigen fragments to the cell surface.

Activation of Helper T Cells (MHC-II)

• Helper T cells have receptors (TCRs) that recognize the antigen-MHC II complex.

• When recognized, the helper T cell activates and releases cytokines.

• Cytokines coordinate the immune response: activating B cells and cytotoxic T cells.

Bridge Between Innate and Adaptive Immunity (MHC-II)

• APCs detect pathogens nonspecifically (innate immunity).

• MHC II presentation triggers specific, adaptive immunity via T cell activation.

Helper T Cells (CD4⁺ T Cells)

FUNCTION: Coordinate and regulate the immune response!

• Recognize antigens presented on MHC II molecules by APCs

• Release cytokines (chemical signals) that:

  • Activate B cells

  • Activate cytotoxic T cells

  • Enhance macrophage activity

• Act like the “managers” of the immune system!

B Cells (B Lymphocytes)

FUNCTION: Produce antibodies and create immune memory!

• Develop in the bone marrow

• Each B cell has a unique receptor for a specific antigen

• When activated (often by helper T cells), they divide and become:

  • Plasma cells → secrete large amounts of antibodies

  • Memory B cells → stay in the body for faster future responses

• Part of humoral immunity (immunity in body fluids)!

Cytotoxic T Cells (CD8⁺ T Cells)

• FUNCTION: Kill infected or abnormal cells!

• Recognize antigens on MHC I (found on most body cells)

• Destroy infected cells using:

  • Perforin → creates holes in the cell membrane

  • Granzymes → trigger apoptosis (cell death)

• Important for fighting viral infections and cancer cells!

Memory Cells (B & T Memory Cells)

FUNCTION: Provide long-term immunity!

• Form after the first exposure to an antigen

• Persist in the body for years (sometimes lifetime)

• Respond rapidly and strongly upon re-exposure

• Basis of vaccination!