Which proteins do CR3 and CR4 receptors recognize?

CR3 (Complement Receptor 3) and CR4 (Complement Receptor 4) are important components of the immune system that play a key role in the recognition of pathogens and the activation of immune responses. These receptors specifically recognize fragments of the complement component C3, which is involved in the opsonization and clearance of pathogens.

The correct answer, which is iC3b, refers to an intermediate form of the complement protein C3 that is generated during the complement activation process. When C3 is cleaved by factors such as C3 convertase, it produces C3b, and upon further cleavage, iC3b is formed. The iC3b fragment still retains the ability to bind to receptors like CR3 and CR4, which enhances phagocytosis and helps immune cells to identify and eliminate pathogens more effectively.

In contrast, C3b is primarily recognized by CR1 and does not bind to CR3 and CR4 as effectively as iC3b. C5a is a potent chemotactic factor but does not interact with CR3 or CR4, and C3d, while important for B cell activation, is primarily recognized by receptors such as CR2, not CR3 or CR