2-3

2. Lymphoid Cells and Tissues

Lymphoid Cells

• Lymphocytes:

  • B cells (produce antibodies), T cells (cell-mediated immunity), and natural killer (NK) cells.

• Macrophages:

  • Derived from monocytes, phagocytose pathogens, dead cells.

  • Antigen-presenting cells (APCs) that activate lymphocytes.

• Dendritic cells:

  • Capture antigens, migrate to lymph nodes to present to T cells.

• Reticular cells:

  • Produce reticular fibers (collagen type III), forming supportive stroma in lymphoid organs.

Lymphoid Tissue

• Diffuse lymphoid tissue:

  • Loosely organized lymphocytes and macrophages in mucous membranes.

• Lymphoid follicles (nodules):

  • Dense, spherical clusters of lymphocytes.

  • Contain germinal centers where B cells proliferate after activation.

3. Lymphoid Organs

Primary lymphoid organs:

• Bone marrow:

  • Site of blood cell production and B cell maturation.

• Thymus:

  • Site of T cell maturation.

  • No follicles, no direct immune response.

Secondary lymphoid organs:

• Lymph nodes, spleen, MALT (mucosa-associated lymphoid tissue): tonsils, Peyer’s patches, appendix.