Defense Against Bacterial Pathogens

The immune defense against pathogenic bacteria involves phagocytes like macrophages from the innate system along with the T and B cells from the adaptive system.

Bacterial pathogens often first encounter the macrophages, which are non-specific and phagocytose diverse bacterial cells.

In contrast, B cells can recognize and internalize only specific bacterial antigens corresponding to their antigen receptor.

Both macrophages and B cells are antigen-presenting cells, or APCs, which process the bacterial antigens and display them using the MHC molecules.

These APCs present the MHC II-bound antigens to naive CD4 T cells, differentiating them into helper T cells. 

These effector cells release cytokines, stimulating B cells to proliferate and differentiate into antibody-producing plasma cells.

The antibody molecules form complexes with antigens and facilitate opsonization — coating the bacterial cells with antibodies — enhancing their recognition and uptake by phagocytic cells.

In infections like tuberculosis, the infected cells present antigens complexed with MHC I to the naive CD8 T cells. They differentiate into cytotoxic T cells, killing the infected cells.