SECTM1B, along with SECTM1A, is a ligand that binds strongly to CD7 [1]. It is involved in modulating T-cell activation, opposing the co-stimulatory effect of SECTM1A, as it proves inhibitory to TCR-mediated T-cell activation [1]. This function may potentially regulate the activation of effector and regulatory T cells, suggesting its importance in the immune response pathways.

SECTM1B was able to compete with SECTM1A for CD7 binding, and both ligands' functional outcomes on T-cell activation were CD7-independent, indicating the existence of alternative receptors [1]. Nucleotide substitution analysis provided evidence for positive selection in SECTM1B, which may be related to its immunological and reproductive functions as suggested by gene expression profiles and homologous sequence annotations [2]. During pneumococcal pneumonia, related Sectm1 genes (including Sectm1b) were induced in conducting airway epithelial cells, with Sectm1a (a related protein) binding to myeloid cells in infected lungs [3].

In conclusion, SECTM1B plays a role in modulating T-cell activation, potentially through alternative receptors. Its positive selection and expression patterns suggest immunological and reproductive functions. In the context of pneumococcal pneumonia, its related genes are induced in airway epithelial cells. The study of SECTM1B using genetic models can help further understand its role in immune-related and reproductive biological processes and potentially in disease conditions such as pneumonia.