Cracking the AP Biology Exam


Animal Structure and Function


In addition to the circulatory system, vertebrates have another system called the lymphatic system. The lymphatic system is made up of a network of vessels that conduct lymph, a clear, watery fluid formed from interstitial fluid. Lymph vessels are found throughout the body along the routes of blood vessels and plays an important role in fluid homeostasis.

The lymphatic system has three functions.

  • It collects, filters, and returns fluid to the blood by the contraction of adjacent muscles.
  • It fights infection using lymphocytes, cells found in lymph nodes.
  • It removes excess fluid from body tissue.

Sometimes a lymph vessel will form a lymph node, a mass of tissue found along the course of a lymph vessel. A lymph node contains a large number of lymphocytes:

Lymphocytes are important in fighting infection. They multiply rapidly when they come in contact with an antigen, or foreign substance recognized by the immune system. (We’ll talk about this in a second.) The lymph nodes swell when they’re fighting an infection. That’s why when you have a sore throat, one of the first things a doctor does is touch the sides of your throat to see if your lymph nodes are swollen, a probable sign of infection.


The immune system, generally speaking, is one of the body’s defense systems. It is a carefully and closely coordinated system of specialized cells, each of which plays a specific role in the war against bodily invaders. As we mentioned above, foreign molecules—be they viral, bacterial, or simply chemical—that can trigger an immune response are called antigens. The appearance of antigens in the body stimulates a defense mechanism that produces antibodies.

The body’s first line of defense against foreign substances is the skin and mucous lining of the respiratory and digestive tracts. If these defenses are not sufficient, other nonspecific defense mechanisms are activated. These include phagocytes (which engulf antigens), complement proteins (which lyse the cell wall of the antigen), interferons (which inhibit viral replication and activate surrounding cells that have antiviral actions), and inflammatory response (a series of events in response to antigen invasion or physical injury).

Types of Immune Cells

The primary cells of the immune system are lymphocytes—T-cells and B-cells. The plasma membrane of cells has major histocompatibility complex markers (MHC markers) that distinguish between self and nonself cells. When T-lymphocytes encounter cells infected by pathogens, they recognize the foreign antigen-MHC markers on the cell surface. T-cells are activated, multiply, and give rise to clones. Some T-cells become memory T-cells, whereas others become helper T-cells. Helper T-cells activate B-lymphocytes and other T-cells in responding to the infected cells. Memory T-cells recognize bacteria or viruses that they have encountered before. Other T-cells, cytotoxic T-cells, recognize and kill infected cells. The activation of T-cells is referred to as a cell-mediated response. T-cells are made in the bone marrow, but mature in the thymus.

In antibody-mediated immunity (humoral immunity), when B-lymphocytes encounter antigen-presenting cells (like macrophages) with foreign MHC markers, they are activated and produce clones. Some B-cells become memory cells that can rapidly divide and can produce plasma cells after an infection has been overcome. These plasma cells produce antibodies that bind to the antigens that originally activated them. Helper T-cells are also involved and produce interleukins. Both memory T- and memory B-cells are responsible for long-term immunity.


AIDS, or “acquired immunodeficiency syndrome,” is a devastating disease that interferes with the body’s immune system. AIDS essentially wipes out the helper T-cells, preventing the body from defending itself. Those afflicted with AIDS do not die of AIDS itself but rather of infections that they can no longer fight off due to their compromised immune systems.

One thing to remember about the immune cells and blood cells: All blood cells, white and red, are produced in the bone marrow. To summarize:

  • T-lymphocytes actually fight infection and help the B-lymphocytes proliferate.
  • B-lymphocytes produce antibodies.


Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the answer that is best in each case. Answers can be found here.

1. All of the following statements about lymph are true EXCEPT

(A) It is fluid that is returned to the blood.

(B) It is derived from interstitial fluid.

(C) It diffuses into tiny lymph capillaries.

(D) It is found within the capillaries of the cardiovascular system.

(E) It contains phagocytes which kill harmful materials in the lymph.

2. B-lymphocytes responding to the HIV surface antigens will

(A) produce proteolytic enzymes

(B) produce antibodies

(C) produce helper T cells

(D) produce macrophages

(E) eliminate body cells infected by the HIV virus

3. All of the following are directly involved in immune responses in mammals EXCEPT the

(A) thymus gland

(B) bone marrow

(C) lymphatic system

(D) spleen

(E) kidney

Directions: Each group of questions consists of five lettered headings followed by a list of numbered phrases or sentences. For each numbered phrase or sentence, select the one heading that is the most closely related to it and fill in the corresponding oval on the answer sheet. Each heading may be used once, more than once, or not at all in each group.

Questions 4–6 refer to the following immune cells

(A) Lymph nodes

(B) Complement proteins

(C) Inflammatory response

(D) Interferon

(E) MHC markers

4. Cell surface proteins that distinguish self from nonself

5. Proteins whose action is nonspecific in the immune system

6. Body’s reaction to pathogen invasion or physical injury