A fundamental tenet of immunology is that adaptive immune responses are initiated in secondary lymphoid tissues. This dogma has been challenged by several recent reports.
A lymph node showing afferent and efferent lymphatic vessels. Image source: Wikipedia, public domain.
Successful T cell-mediated immunity can be initiated outside of such dedicated structures (lymph nodes - secondary lymphoid tissue), whereas they are required for adaptive humoral immunity.
This resembles an ancient immune pathway in the oldest cold-blooded vertebrates, which lack lymph nodes and sophisticated B-cell responses including optimal affinity maturation.
The T-cell, however, has retained the capacity to recognize antigen in a lymph node-free environment.
B Cell Types (click to enlarge the image).
What chemokine attracts naïve B cells to lymph nodes?
CXCR5. CXCL13 binds to CXCR5.
What chemokine attracts naïve T cells to lymph nodes?
CCR7. CCL19 and CCL21 bind to CCR7.
B-cells need a proper house, whereas T-cells are happy in a cave: the dependence of lymphocytes on secondary lymphoid tissues during evolution. Janin Hofmann et al. Trends in Immunology, 2010.