Briefly describe the terms "antigen" and "antibody", and describe the structure of the latter. Briefly describe the processes of opsonisation, phagocytosis and destruction of bacteria by macrophages.

An “antigen” is a specific foreign molecular entity which is bound by antibodies during an immune response. Examples of antigens include peptides from bacterial surface proteins, which signal the presence of this pathogen to the adaptive immune system. An “antibody” is a y-shaped, B-Cell-secreted molecule composed of variable and constant regions along a heavy and light chain. It can be divided into “Fc” (“Fragment Crystallisable”) and “Fab” (“Fragment antigen-binding”) regions, the latter of which contain highly specific binding sites for one particular antigen.
“Opsonization” is the process of coating a foreign invader in antibody, to facilitate processes such as phagocytosis. When a bacterium is opsonized, antigens on its surface are bound by the Fab regions of antibodies, leaving the Fc regions at the tail end exposed to the immune system. When a phagocyte such as a macrophage binds these Fc regions by the Fc receptors on its surface, it begins engulfing the opsonized bacterium by “ruffling” its membrane around it, essentially using the opsonizing antibodies as “grips” to “grab” onto the bacterium. This leads to the formation of a vesicle that detaches from the macrophage membrane and enters the cytoplasm for further action, called the “phagosome”. This phagosome is then fused with another type of vesicle called “lysosomes”, which contain agents that will degrade the bacterium, such as reactive oxygen species and hydrolytic enzymes. The new vesicle resulting from this fusion is called the “phagolysosome”, which kills and breaks down the captured pathogen.