This lecture covers the recognition of antigens by B and T cells, focusing on the two major strategies for antigen recognition by B lymphocytes: thymus-independent type 1 (TI-1) and thymus-independent type 2 (TI-2). It explains how B cells generate short-lived IgM responses without memory, and how T cells recognize antigen fragments. The lecture also discusses the role of dendritic cells as the only antigen-presenting cells capable of activating naive T cells.