Immune processes

MHC (Major Histocompatibility Complex)

• MHC = membrane-bound glycoproteins

• Present antigens to T cells via T-cell receptor (TCR)

MHC Class I

• Expressed by all nucleated cells

• Presents to CD8+ Cytotoxic T Cells

• Shows endogenous (intracellular) antigens

MHC Class II

• Expressed only by APCs (DCs, B cells, macrophages)

• Presents to CD4+ Helper T Cells

• Shows exogenous (extracellular) antigens

Phagocytes and Antigen Presentation

• Migrate from infection site → lymphoid tissue

• Present antigen to naïve helper T cells via MHC-II

• Dendritic cells (DCs): Best at activating naïve T cells

• Macrophages: Less effective at priming naïve T cells; important for secondary responses

B Cell Activation

• B cell binds antigen via BCR

• Endocytosis of receptor-antigen complex

• B cell presents processed antigen on MHC-II

• CD4+ Helper T cell recognizes antigen

• Cytokines from helper T cell activate B cell

• B cell → Plasma cells (antibodies) + Memory B cells

Humoral Immune Response (Antibody-Mediated)

1. Macrophage/DC phagocytoses pathogen → presents via MHC-II

2. Helper T cell recognizes antigen via TCR and CD4

3. B cell processes and presents antigen via MHC-II

4. Helper T cell binds B cell → releases cytokines

5. B cell fully activated → Clonal expansion

6. Plasma cells → secrete antibodies

7. Memory B cells ensure long-term immunity

   • Outcome: Antibody production, neutralization, opsonization, and complement activation

Complement System (Part of Humoral Immunity)

Enhances pathogen clearance via:

• Opsonization

• Inflammation

• Membrane Attack Complex (MAC) → Cell lysis

Complement Activation Pathways: Classical

Triggers antibody-antigen complex

Complement Activation Pathways: Lectin

Mannose-binding lectin binds pathogen

Complement Activation Pathways: Alternative

Pathogen surface molecules

Cell-Mediated Immune Response

• Cytotoxic T Cells (CD8+)

1. Infected cell presents antigen on MHC-I

2. CD8+ CTL recognizes antigen via TCR

3. CD8 binds MHC-I, T cell activated

4. Perforin forms pores in target cell membrane

5. Granzymes enter → trigger apoptosis

Interleukin-2 (IL-2) from helper T cells

• Promotes CTL proliferation

• Helps B cells switch antibody types

Memory & Coordination

• Helper T cells orchestrate immune responses

  • Activate B cells → Humoral response

  • Support CD8+ T cells → Cell-mediated response

• Memory cells persist long-term → faster, stronger response upon re-exposure