-     Innate immunity: Defenses against any pathogen

-     Adaptive immunity: Induced resistance to a specific pathogen


Dual Nature of Adaptive Immunity

-     T and B cells develop from stem cells in red bone marrow

-     Humoral immunity

-     B cells mature in the bone marrow

-     Cellular immunity

-     T cells mature in the thymus



-     Antigen (Ag): A substance that causes the body to produce specific antibodies or sensitized T cells

-     Antibodies (Ab) interact with epitopes or  antigenic determinants

-     Hapten: A small molecule that acts antigenic once it has been combined with carrier molecules (to form a hapten-carrier conjugate)



-     Globular proteins called immunoglobulins

-     The number of antigen-binding sites determines valence

-     Most antibodies have two binding sites, and are called bivalent (two binding sites) monomers

-     Some antibodies have 2 subunits (dimers) or 5 subunits (pentamers)


B Cells and Humoral Immunity

-     B cell surface immunoglobulin binds with an antigen

-     Antigen is internalized and processed

-     Antigen fragment combines with a Major Histocompatibility Complex (MHC)

-     MHC with Ag fragment is displayed on cell surface

-     T-dependent antigens bind to MHC/Ag fragment

-     TH cell produces cytokines that activate the B cell

-     T-independent antigens

-     Stimulate the B cell to proliferate as plasma cells that make Abs


Activation of B Cells

-     B cells differentiate into

-     Antibody-producing plasma cells

-     Memory cells

-     Clonal deletion eliminates harmful B cells

-     Humoral response is useful against viruses and bacteria that are circulating freely in the body


T Cells and Cellular Immunity

-     T cells mature in the thymus

-     Thymic selection eliminates many immature T cells

-     T cells respond to Ag by T-cell receptors (TCRs)

-     T cells require antigen-presenting cells (APCs) to be able to recognize Ag

-     B cells and macrophages often act as APCs

-     Many pathogens that enter the gastrointestinal or respiratory tracts pass through epithelial cell gateways

-     M (microfold) cells in the intestines

-     PeyerŐs patches, which contain APCs


T Cells


T Helper cells (CD4+ or TH cells)

-     Help B cells that are acting as APCs produce antibodies

-     Activate macrophages that are acting as APCs

-     TH cells produce cytokines and differentiate into

-     TH1

-     TH2

-     Memory cells




T Cytotoxic Cells (CD8+ or TC cells)

-     Target cells are self cells carrying endogenous antigens

-     Activated into cytotoxic T lymphocytes (CTLs)

-     Require TH, dendritic cell, and stimulatory signals

-     CTLs recognize Ag + MHC I

-     Induce apoptosis in target cell

-     CTL releases perforin and granzymes


T Regulatory Cells (Treg cells)

-     CD4 and CD25 on surface     

-     Suppress T cells against self


Antigen-Presenting Cells

-     Digest antigen

-     Ag fragments on APC surface with MHC

-     B cells

-     Dendritic cells

-     Activated macrophages


Natural Killer (NK) Cells

-     Granular leukocytes destroy cells that donŐt  express MHC I

-     Kill virus-infected and tumor cells

-     Attack parasites



-     Antibody-dependent cell-mediated cytotoxicity



-     Chemical messengers

-     Overproduction leads to cytokine storm


Immunological Memory

-     Antibody titer is the amount of Ab in serum

-     Primary response occurs after initial contact with Ag

-     Secondary (memory or anamnestic) response occurs after second exposure





Types of Adaptive Immunity

-     Naturally acquired active immunity

-     Resulting from infection

-     Naturally acquired passive immunity

-     Transplacental or via colostrum

-     Artificially acquired active immunity

-     Injection of Ag (vaccination)

-     Artificially acquired passive immunity

-     Injection of Ab


Terminology of Adaptive Immunity

-     Serology: The study of reactions between antibodies and antigens

-     Antiserum: The generic term for serum because it contains Ab

-     Globulins: Serum proteins

-     Immunoglobulins: Antibodies

-     Gamma (g) globulin: Serum fraction containing Ab