Cells of Immune System• Lymphocytes

– B lymphocytes produce antibodies• Mostly located in germinal centers of lymphatic organs

– T lymphocytes provide cell-mediated immunity• 65 – 85% circulating lymphocytes are T cells, many others are

in cortical areas of lymph nodes

• Antigen presenting cells (APCs)– involve phagocytosis, antigen processing, and

presentation of antigen on cell’s surface• Mostly remain in lymphoid tissues such as lymph node sinuses

or they may be fixed as in the liver (Kuppfer cells)

Initial Immune Response

T CELLSRECOGNITION OF ANTIGEN

Initial Immune Response

• Requires antigen recognition by a T lymphocyte (T cell recognition)

• Two groups of antigens– Endogenous antigens– Exogenous antigens

Endogenous Antigens

Endogenous Antigens

• Proteins that are produced within a cell– Include all viral proteins– Include proteins from mutant genes (cancer cells)

Endogenous Antigens

• Antigenic proteins manufactured by cell (such as viral proteins)

• Display of fragments on cell surface within a Class I Histocompatibility Molecule (MHC I)

Class I Pathway

Class I Histocompatibility Molecules• Are found on all body cells• Are transmembrane proteins• Are bound by a T cell that can

– Recognize antigenic peptide and flanking histocompatibility molecule

– CD8 molecules (CD8 T cell) that bind to CD8 receptor

Recognition is by CD8 T cell (cytotoxic T cell).

Cytotoxic T cells recognize

• Molecules on surfaces of virus infected cells

• Some tumor cells• Tissue transplant• Need

costimulation by cytokines produced by helper T cells

Exogenous Antigens

Exogenous Antigens

• Exogenous antigens are those from “outside the body.”– Include those inhaled, ingested, or injected

• Must be taken up by antigen presenting cell (APC)– Phagocytic cells that include dendritic cells and

macrophages

Phagocytic cells (APCs)

• Endocytosis of antigen• Digestion of antigenic substance into

fragments• Display of fragments on cell surface within

a class II histocompatibility molecule• Recognition by CD4 T cell (helper T cell).

Class II Pathway

Class II Histocompatibility• Are found on phagocytes such as dendritic and

macrophages• Are transmembrane proteins• Are bound by a T cell that can

– Recognize antigenic peptide and flankinghistocompatibility molecule

– CD4 molecules (on CD4 T cell) that bind to CD4 receptor

Recognition is by CD4 T cell (helper T cell).

Helper T cells recognize

• Molecules on surfaces of APC (macrophages)

• Costimulation to activate cytotoxicT cells

Initial Immune Response

B CELLSRECOGNITION OF ANTIGEN

B Cell Antigen Processing

• Follow MHC II pathway• Processing of antigen differs by

– Receptor-mediated endocytosis (not phagocytosis)– Receptors for antigen are antibodies located in

plasma membrane– May have high affinity for antigen (more efficient

than macrophage consuming antigen)– Following steps are same MHC II pathway

B Cell Antigen Processing

• Antigen fragments displayed on MHC II molecule.

• CD 4 T cell bind to MHC II of B cell (recognizes antigen) and releases lymphokines (CD 4 T cell is “helper T”)

• Lymphokines cause B cells to enter cell mitotic cycle

• B cells clone (plasma cells)• Release antibodies (are same as receptors but

without membrane attachment)