The correct option is D Both a and b
Immune system defends our body against any disease causing agents. It attacks anything which seems to be foreign and removes it from the body. For example, during kidney transplant, if the donor kidney does not match the recipient then the recipient’s immune system considers the graft as foreign and rejects it. This process is called graft rejection.
Although the immune system attacks only foreign agents, sometimes it may mistake it’s own body cells as foreign agents and pose an immune response against them. Such a response can be fatal and cause damage to the body. For example, attack of immune system on glomerulus of kidney can cause glomerulonephritis (inflammation of glomerulus).
Renal calculi refers to the stone or insoluble mass of crystallised salts formed within the kidney.