Cellular and Humoral Immune Responses: Mechanisms & Activation

Cellular Immune Response

The cellular immune response occurs against: microorganisms with intracellular growth and development (bacteria, fungi, viruses, protozoa); cells that are foreign to an organism from another individual (e.g., transplanted organs); and tumor cells. This response involves cytotoxic T cells and helper T cells, which collaborate with antigen-presenting macrophages.

Activation Process

Recognition of Antigen: T lymphocytes recognize antigens only when they are bound to MHC molecules on cell surfaces. When a macrophage detects an extracellular antigen, it activates, phagocytoses the antigen, and processes its proteins into linear peptides. These peptides bind to MHC class II molecules, which are then transported to the macrophage surface. If the antigen is from an intracellular pathogen, some peptides bind to MHC class I molecules and are transported to the cell surface.

Lymphocyte Activation

Helper T Lymphocytes: These recognize MHC class II molecules on macrophages, which activates them and stimulates the release of interleukin-1 by the macrophages. Activated helper T cells produce interleukin-2, which causes proliferation of helper T cells, cytotoxic T cells, and macrophages, facilitating the destruction of the antigen.

Cytotoxic T Lymphocytes: These recognize MHC class I molecules on cells. After recognition, the cells become active and proliferate due to interleukin-2 secreted by helper T cells (Th2). Cytotoxic T cells adhere to the target cell and cause its destruction. This occurs because the union stimulates the target cell to release perforin, which creates pores in the cell membrane, causing lysis of the target cell. The remaining cell fragments are then digested by macrophages.

Humoral Immune Response

The effector elements are antibodies, which act against the antigens that caused their formation. This response involves B lymphocytes (which produce antibodies) and a group of helper T lymphocytes (Th2). The response can occur in two ways: humoral response involving both B lymphocytes and helper T cells (common) or involving only B lymphocytes.

B Lymphocyte and Helper T Cell Response

Activation depends on the recognition of antigens by B lymphocytes via their surface receptors (antibodies) and the interaction with Th2 lymphocytes.

Process

Helper T cells recognize MHC class II molecules on cell surfaces. Macrophages with complement receptors attached to antigens bind to them, activate, and proliferate. An inactivated B lymphocyte recognizes its antigen, which activates a helper T cell. The B lymphocyte then endocytoses the antigen and presents some of the resulting peptides bound to MHC class II molecules. The B lymphocyte becomes an antigen-presenting cell to activated Th2 cells. The helper T cell recognizes the MHC II-peptide complex on the surface of the B cell, binds, and secretes interleukins. This activates the B lymphocyte, causing production and proliferation, creating a clone of B cells that differentiate into plasma cells (which secrete antibodies to bind and neutralize or mark antigens) and memory B lymphocytes.