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.

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 PresentationInitial 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