Adaptive/Acquired Immunity DENT 516

Adaptive immunity Refers to antigen-specific defense mechanisms that take several days to become protective and are designed to remove a specific antigen. This is the immunity one develops throughout life. There are two major branches of the adaptive immune responses: humoral immunity and cell-mediated immunity.

Cell Mediated Immunity Humoral Immunity humoral immunity involves the production of antibody molecules in response to an antigen and is mediated by B-lymphocytes. Cell Mediated Immunity CMI involves production of cytotoxic T-lymphocytes, activated macrophages, activated NK cells and cytokines in response to an antigen and is mediated by T-lymphocytes.

Antigens Ags are molecules that elicit an immune response in the body Ags can be: Proteins Polysaccharides Conjugates of lipids with Proteins (lipoproteins) Polysaccharides (glycolipids)

Antigens II Ags that enter body from environment include: Inhaled macromolecules; e.g., cat hair proteins asthma Ingested macromolecules; e.g. shellfish proteins allergy Molecules introduced beneath the skin; e.g. splinter, vaccine Exogenous antigens Ags generated within the cells of the host: Proteins encoded by viral genes that have infected a cell Aberrant proteins that are encoded by mutant genes; e.g. proteins from mutated genes in cancer cells Endogenous antigens

Antigen Presentation Initial immune response to any Ag requires the Ag be recognized by a T cell. This is best exemplified by AIDS w/loss of CD4+ T cells. Two categories of Ags are processed and presented to T cells by different mechanisms: Exogenous Ags Endogenous Ags Dendritic cell B cell T cell activated; cytokines released; Th1, activate M; Th2, activate B cells

Exogenous antigens E. Ag’s (inhaled, ingested, injected), taken up by APCs: Phagocytic cells; dendritic cells, macrophages B lymphocytes (produce antibodies) • APCs  engulf Ags by endocytosis (endosome-lysosome)  Ag degraded into short peptides  peptides displayed at cell surface nestled w/i a class II histocompatibility molecule  recognized by CD4+ T cells

Endogenous Antigens Ags that are generated w/i a cell; e.g., Mt bug, viral proteins in infected cells Peptides displayed at cell surface nestled w/I a class I histocompatibility molecule Recognized by CD8+ T cells CD8+ T cells are cytotoxic Have machinery to destroy infected cell T cell activated; kills infected cell

B Lymphocytes: A Special Case Process Ag by MHC II pathway but: B cells engulf Ag by receptor mediated endocytosis BCRs are surface antibodies anchored in plasma membrane Affinity of BCR for an Ag epitope is so high that the B cell can internalize the Ag at concentrations thousands of times smaller than needed for a macrophage CD4+ T cell recognizes displayed Ag and is stimulated to release cytokines These stimulate B cells to grow into a clone of cells; plasma cells These plasma cells synthesize BCRs with identical binding site for the Ag epitope but w/o the transmembrane tail The Abs are secreted release Th2 cytokines

Clonal Expansion

Types of T cells