Defective humoral responses in CD19Tln1^−/− mice. (A) IgG, IgM, and IgA serum levels in naive WT or CD19 Tln1^−/− mice (talin null). Results are the mean ± SEM of 10 mice in each experimental group. The differences between the compared experimental groups were statistically insignificant. (B) Absolute numbers of B cells (i, B220⁺) or T cells (ii, CD3⁺) in the footpad draining peripheral lymph nodes of either WT or CD19Tln1^−/− mice, either naive or 10 days after primary immunization with the T-dependent antigen, KLH-TNP (KLH-TNP28). **P < .002. n = 2. (C) Antigen- and hapten (TNP)–specific IgG and IgM titers, expressed as arbitrary units, after immunization with the T-dependent antigen, KLH-TNP (KLH-TNP28). CD19Tln1^−/− (labeled as talin null) and WT mice were each immunized in the footpad. After 2 and 4 weeks, mice received additional antigenic boosts into their peritoneal cavity; and 2 weeks later, serum was analyzed for KLH, KLH-TNP, or BSA-TNP specific antibodies by enzyme-linked immunosorbent assay. Results are mean ± range and are expressed as arbitrary units. *P < .05. **P < .01. n = 2. Top panel: Serum IgG reactivity against KLH only, KLH-TNP28, or BSA conjugated with TNP at either low or high density (TNP32 and TNP3, respectively). Bottom panel: Serum IgM reactivity against KLH only, KLH-TNP28, or BSA carrying either high or low density of TNP (TNP32 and TNP3, respectively).