Read the passage carefully and choose the best answer to each question out of the four alternatives and click the button corresponding to it. In the context of the immunology passage, which statement about antigen–antibody immunity is factually true according to the text?

Introduction / Context:
This question again uses the same immunology passage but now asks the learner to identify which statement is true according to the text. Such questions test comprehension of specific details and the ability to distinguish between correct and incorrect factual statements. The passage gives several technical terms, including lysis, phagocytosis, antigen, antibody, and complement, and the options are built around these definitions and descriptions.

Given Data / Assumptions:

• The passage states that antigen–antibody reaction was regarded as typical of immunological responses.
• It defines antibodies as proteins synthesized by plasma cells when antigens contact lymphocytes.
• It explains that lysis is rapid physical rupture of antigenic cells, executed by complement.
• Complement is described as a complex and unstable blood constituent that must be activated by a specific antibody.
• Phagocytosis involves antigenic particles being engulfed and often digested by macrophages and polymorphs.

Concept / Approach:
The concept tested is recognition of accurate factual information versus distractors. We need to restate in our own words what the passage says about each term, and then check which answer option matches exactly. Any option that introduces a psychological angle, changes the nature of antigen, or misdefines lysis or phagocytosis must be eliminated. The only safe choice is the one that corresponds closely to the sentences of the passage without distortion or exaggeration.

Step-by-Step Solution:
Step 1: Recall or re read how the passage describes complement. Step 2: The passage says that the process of lysis is executed by a complex and unstable blood constituent known as complement. Step 3: Compare this description with Option C, which states that complement is a complex and unstable constituent of blood. Step 4: Note that the wording matches almost exactly, so Option C is very likely correct. Step 5: Examine Option A, which calls antigen antibody a psychological process. The passage clearly talks about immunological, not psychological, reactions. Step 6: Examine Option B, which incorrectly defines lysis as forming plasma cells, while the passage defines it as physical rupture of antigenic cells. Step 7: Examine Option D, which calls antigen a natural part of the organism body, but the passage clearly says antigen is a substance foreign to the body. Step 8: Examine Option E, which wrongly associates phagocytosis with antibodies breaking down, although the passage states it involves macrophages engulfing antigenic particles. Step 9: Conclude that Option C is the only statement fully supported by the passage.

Verification / Alternative check:
To verify, we match each term used in the options with its passage definition. Lysis involves rupture of antigenic cells, not formation of plasma cells. Antigens are foreign substances, so Option D is wrong. Phagocytosis deals with engulfing antigenic particles, not decomposing antibodies. The passage never connects antigen antibody reactions to psychology, so Option A is also wrong. This leaves only the statement about complement being a complex and unstable blood constituent, which fits exactly. This cross elimination technique confirms our answer.

Why Other Options Are Wrong:
Antigen–antibody is described as a psychological process of the mind is wrong because the passage deals with immunological, physiological mechanisms. Lysis is a process of forming plasma cells is wrong because plasma cells are formed from lymphocytes, while lysis refers to physical rupture of antigenic cells. Antigen is part of the organism body is incorrect because the passage calls it a substance foreign to the body. Phagocytosis is the process in which antibodies break down into smaller molecules is also incorrect because the passage states that macrophages engulf antigenic particles, not antibodies themselves.

Common Pitfalls:
Students often skim scientific passages and remember only isolated technical words, then guess based on vague memory. This can make any scientific sounding option feel correct, even if it contradicts the text. Another common pitfall is to rely on partial personal knowledge of biology and ignore the specific wording of the passage. In competitive exams, the safest method is to trust the passage more than memory and to verify each option directly against what is written.

Final Answer:
The correct answer is Complement is described as a complex and unstable constituent of blood.