37. Kinetics of the immune response. Humoral and cellular immunity. Primary and secondary immune response. Immunological memory · Human Biology · MU Plovdiv

Transition from Central to Peripheral Organs

B and T lymphocytes complete their development and become immunocompetent in the central lymphoid organs (bone marrow for B cells and thymus for T cells).
These immunocompetent cells then migrate to peripheral lymphoid organs where they participate in immune responses upon encountering antigens.

These organs are the primary sites for initiation and regulation of immune responses.
Lymphocytes in these organs become activated upon recognizing antigens.
Activated lymphocytes undergo clonal proliferation and antigen-specific differentiation into effector cells (that eliminate pathogens) and memory cells (that persist for future immune responses).
This process is antigen-dependent, unlike their earlier maturation which is antigen-independent.
Effective immune responses depend on interactions between B cells, T cells, and antigen-presenting cells, all coordinated by signaling molecules called cytokines.

Lymph nodes are encapsulated and divided into three main areas:
Cortex: Contains B cell follicles. Primary follicles are inactive while secondary follicles contain germinal centers where B cells actively proliferate and differentiate.
Paracortex: Rich in T cells. This is where T cells are activated during cellular immune responses.
Medulla: Houses plasma cells that secrete antibodies.
Lymph nodes filter lymph fluid draining from tissues, allowing antigens from infections or injuries to be presented to lymphocytes.
Specialized blood vessels called High Endothelial Venules (HEVs) allow naïve lymphocytes to enter the node from the blood.
Once activated, lymphocytes exit through efferent lymphatic vessels and re-enter the bloodstream to reach affected tissues.

The spleen deals with antigens circulating in the blood, especially those from blood-borne infections.
Structurally divided into:
Red pulp: Involved in filtering blood and removing old red blood cells.
White pulp: Contains immune cells. It surrounds arterioles and includes:
Periarteriolar lymphoid sheaths (PALS): T cell zones.
Marginal zones: B cell areas.

MALT protects mucosal surfaces like the respiratory, digestive, and genitourinary tracts.
It lacks a capsule and includes tissues such as:
Tonsils
Adenoids
Peyer’s patches
Appendix
MALT primarily supports IgA-mediated immunity, which is crucial for mucosal defense.
Some B cells activated in MALT migrate to the mammary glands, where they secrete IgA antibodies into breast milk, offering passive immunity to infants.