Membrane attack complex (MAC) is seen associated with
A. B-lymphocytes
B. Complement-system
C. Macrophages
D. T-lymphocytes

Hint: MAC is also known as terminal complement complex. This structure is formed on the pathogen cell membrane as a result of the activity of the host alternative pathways. This structure leads to lysis or death of the pathogen.

Complete Answer:
- The complex between complement system proteins is the key for the assembly of the membrane attack complex .The effector proteins of the immune system are involved in the formation of MAC. The MAC complex forms transmembrane channels which lead to cell lysis and death of the pathogen.
- Mac is composed of 4 types of complement system proteins that can bind with the outer surface of the plasma membrane. The other proteins help in forming a ring in the membrane. This ring structure is a pore that allows free diffusion of molecules in and out of the cell. More number of pores leads to death of the cell.

(I) B-lymphocytes- It is a type of WBC. It secretes antibodies in response to antigens. It is responsible for the body's adaptive immunity.
(II) Macrophages are large cells which recognise, engulf and destroy the infectious cell or damaged or dead cells.
(III) T-lymphocytes- These are the type of WBC. They are derived from stem cells. It protects the body from infection and is helpful in fighting cancer.

The correct answer is (B). MAC is associated with the complement system.

Note: MAC is formed as a result of the abnormal activation of the complement system. It brings about a series of reactions in which the blood proteins come together, binds them to the cell wall and enter the cell membrane. It quickly results in cell death due to movement of water and ions in and out of the cell.