Adaptive immunity

Key features

• Antigen-specific

• Not innate – develops after exposure to foreign material

• Slow initial response – 5–6 days

• Memory – rapid, strong response upon re-exposure

• Lifelong protection (mostly)

• Only in vertebrates

• Involves lymphatic system

Lymphocytes (B cells & T cells)

• Size: ~6 μm diameter

• Lifespan: 3 days to 8 weeks

• Circulation: Blood & lymph

• Activation: By antigen recognition

• Origin: Both from bone marrow

Primary Lymphoid Organs

• Bone Marrow: B cell maturation, origin of T & B cells

• Thymus: T cell maturation

Secondary Lymphoid Organs

• Lymph nodes

• Spleen

Lymph nodes

• Filters lymph

• Contains dendritic cells, macrophages (phagocytosis)

• Activation site for lymphocytes

Spleen

• Filters blood

• Germinal centers → rapid B cell proliferation

Lymphatic System Flow

• Interstitial fluid & cells enter lymphatic vessels

• Lymph flows to lymph nodes

• Pathogens activate APCs (Antigen Presenting Cells)

• Lymph returns to blood via large ducts

Antigen

Any foreign molecule eliciting lymphocyte response

Epitope

Specific region of an antigen recognized by immune cells

Receptors (BCR/TCR)

• Specific to 1 epitope

• Polypeptides in plasma membrane

• Binding via variable region

Humoral

• Mediated by B cells → Antibodies

• Targets extracellular pathogens & toxins

Cell-Mediated

• Mediated by Cytotoxic T cells (CD8)

• Targets infected, cancer, and transplanted cells

Antigen Presenting Cells (APCs)

• Bridge innate & adaptive immunity

• APCs include:

  • Dendritic cells

  • Macrophages

  • B cells

Cytotoxic T Cells (CD8+)

• Kill infected, cancer, or transplanted cells

• Recognize antigens with MHC Class I

Helper T Cells (CD4+)

• Coordinate immune response

• Recognize antigens with MHC Class II

Antibodies (Immunoglobulins)

Structure:

• Variable regions bind antigen

• Heavy & light chains linked by disulfide bonds

Antibody Classes

Ig.. MEDAG

IgM

First antibody produced; pentameric

IgE

Allergy & parasitic response

IgD

Membrane-bound on B cells

IgA

Found in secretions (e.g., breast milk)

IgG

Most abundant in circulation