Adaptive Immune Response Overview

Adaptive Immune Response

• Highly specialized, specific defense mechanism for recognizing and fighting foreign pathogens and abnormal cells
• Slower than innate immunity, but has a longer-lasting effect

Specificity

• Targets specific pathogens/abnormal cells based on unique antigens
• Has a memory component, creating memory cells to recognize and respond to an antigen more rapidly if encountered again

Self-Tolerance

• Differentiates between "self" and "non-self" to prevent attacking the body's own cells (autoimmunity)

Antigen Presentation

• Foreign substances presented by antigen-presenting cells (APCs)

• Examples include dendritic cells, macrophages, and B cells

• APCs capture, process, and present antigens to T lymphocytes via major histocompatibility complex (MHC) molecules

• MHC Class I molecules present antigens from intracellular pathogens (e.g., viruses) to CD8+ cytotoxic T cells.

• MHC Class II molecules present antigens from extracellular pathogens (e.g., bacteria) to CD4+ helper T cells.

Humoral Immunity

• Occurs in bodily fluids (e.g., blood, lymph)
• TH cells release cytokines that activate B cells
• Clonal expansion of B cells leads to plasma cells and memory B cells
• Plasma cells produce large quantities of antibodies.
• Antibodies neutralize pathogens, inhibiting their entry into host cells, agglutinating pathogens, activating complement, and enhancing phagocytosis.
• Antibodies can act as opsonins (mark pathogens for phagocytosis).

Cell-Mediated Immunity

• Happens within infected cells
• Dendritic cells, macrophages, and B cells present antigens to T cells on the cell surface via MHC molecules.
• If MHC Class II is present, CD4+ T cells are activated releasing cytokines (e.g., interleukin-2) that enhance the activity of cytotoxic T cells, macrophages, and other immune cells. These cells undergo clonal expansion, producing many T cells specific to the antigen.
• Cytotoxic T cells directly kill infected or abnormal cells by recognizing specific antigens presented by MHC Class I molecules.
• Perforin creates pores in the cell membrane, releasing granzymes to induce apoptosis.
• Natural Killer cells can help target cells missing MHC Class I molecules or with abnormal patterns.
• Activated macrophages are stimulated by cytokines to become more effective at engulfing and destroying pathogens.