For us to combat the enormously diverse and numerous environmental microbial pathogens, our immune system has evolved two arms: thymus-derived T lymphocytes for cellular immunity and bone marrow-derived B lymphocytes for humoral immunity. Ever since these two important cells were discovered, there has been appreciation that T lymphocytes also provide essential instruction for B cells to produce large numbers of high affinity antibodies. B and T cell collaboration in immune responses leading to protective memory and affinity matured antibody responses occurs in the hallmark structure called germinal center, where B cells in response to T cell help proliferate extensively and undergo somatic hypermutation in their antibody genes. Dysfunction of germinal center reactions are at the core of various diseases, including immunodeficiency, lymphoma and autoimmunity. Understanding the cellular and molecular regulation of germinal center reactions is thus of high clinical relevance and will benefit the design of vaccines against infectious diseases, including HIV. The long-term goal of this GRC series is to understand how T cells regulate humoral immunity, with a particular emphasis on the germinal center reactions. A subset of T lymphocytes has been found to express the chemokine receptor CXCR5 and to localize in germinal centers. These cells have been named as B follicular helper T (Tfh) cells and shown to play critical roles in germinal center reactions. New exciting findings by the organizers of this meeting as well as many invited speakers are starting to elucidate the unique genetic programming and biological features of these cells, in comparison with other types of T cells- including TH1, TH2, TH17 and regulatory T cells. The goal of the conference is to facilitate exchange of the latest basic scientific findings and promote discussion on the role of Tfh in a wide range of diseases. Although the GRC title is on Tfh cells as a timely topic to draw attention from immunology community and to have distinction from other meetings on B cells and other types of T cells, we will have additional discussion on other types of T cells including effector, memory and regulatory T cells in humoral immunity and we will cover the regulation on B cells in germinal center reactions.