The adaptive immune system is comprised of two interrelated immune pathways – humoral and cell-mediated immunity
- Both pathways involve elements of the innate immune system to coordinate their respective immune reactions
Humoral Immunity
Humoral immunity describes the pathway by which antibodies are produced by B lymphocytes to target exogenous antigens
- When macrophages engulf exogenous pathogens, they digest them within lysosomes to release antigenic fragments
- These fragments are presented on special surface receptors (MHC class II) that denote the material as being foreign
- The antigens are presented to helper T cells, which in turn secrete cytokines to activate the appropriate B lymphocytes
- The specific B lymphocytes divide and differentiate (clonal selection) to form antibody producing plasma cells
Cell-mediated Immunity
Cell-mediated immunity describes a pathway that does not result in antigen production but instead targets endogenous antigens
- Cancerous and virus-infected cells involve the body’s own cells and thus are not recognised as foreign, evading normal detection
- These cells may instead present antigenic fragments as a complex with their own self markers (MHC class I)
- When helper T cells identify these cells, they stimulate a second type of T lymphocyte – cytotoxic T cells (TC cells)
- Cytotoxic T cells show specificity to particular antigens and will bind to the presented antigen and release perforating enzymes
- These enzymes cause the infected / cancerous cell to by lysed, preventing the further spread of infection
- Virus infected cells can also be destroyed non-specifically by NK cells, which respond to interferon released by the infected cell
Summary of Adaptive Immune Pathways
