Question

Process of antibody formation is a slow-acting, long-lasting, and more specific adaptive immune response. The adaptive immune response in B cells, Helper T cells and Cytotoxic T cells involve four phases mentioned below. Arrange these phases according to the order of occurence from first to last.
(A) Activation: Activation responses involve both changes in gene expression and in the initiation of cell division.
(B) Memory: Long-lived, pre-activated immune cells that wait for a subsequent infection are formed in the memory phase of the adaptive immune response.
(C) Encounter: Cells must first encounter antigens by binding specifically to them using specialized membrane proteins.
(D) Attack: The immune cells attack invading pathogens or infected cells.
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (D), (C), (A), (B).
• C. (B), (A), (D), (C).
• D. (C), (A), (D), (B).

Hint:

To remember the sequence of adaptive immunity:
You must first Encounter the enemy, then get Activated for battle, carry out the Attack, and finally preserve the Memory of the fight.
This gives the order: (C), (A), (D), (B).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The adaptive immune response is a highly coordinated process that involves the clonal selection, expansion, and differentiation of antigen-specific lymphocytes (B and T cells).
This response follows a logical physiological sequence from the initial entry of a foreign antigen to the establishment of immunological memory.

Step 2: Detailed Explanation:
Let us trace the four phases of the adaptive immune response in order:
1. Encounter (C): The process begins when naive B or T lymphocytes, migrating through lymphoid tissues, first encounter their specific foreign antigen.
This encounter is mediated by specialized membrane-bound antigen receptors (B-cell receptors/antibodies or T-cell receptors) that bind specifically to the antigen.
2. Activation (A): Once the antigen is bound, and the lymphocytes receive necessary co-stimulatory signals, they enter the activation phase.
This phase involves changes in gene expression, leading to metabolic activation, protein synthesis, and clonal expansion through rapid cell division.
Activated cells differentiate into effector cells (such as antibody-secreting plasma cells, helper T cells, or cytotoxic T cells).
3. Attack (D): The newly generated effector lymphocytes execute their specific defense mechanisms.
B-cell-derived antibodies neutralize toxins and opsonize pathogens; helper T cells secrete cytokines to direct the immune response; and cytotoxic T cells target and destroy virus-infected or abnormal host cells.
4. Memory (B): After the infection is resolved, the majority of effector cells undergo apoptosis.
However, a small fraction survives and differentiates into long-lived, quiescent memory B and T cells.
These memory cells persist in the body, ready to mount a rapid, robust secondary immune response if the same pathogen is encountered again.
Therefore, the correct chronological sequence is (C) \(\rightarrow\) (A) \(\rightarrow\) (D) \(\rightarrow\) (B).

Step 3: Final Answer:
The correct option is (D).