Grade 10-11 (age 15-17)

high school Defense — Immunity

Immune System

When pathogens get in, the body first uses nonspecific defenses such as skin, mucous membranes and phagocytes, which meet every pathogen the same way, and then specific defenses, which single out particular antigens. Specific defense includes humoral immunity, in which antibodies made by B lymphocytes bind to antigens, and cell-mediated immunity, in which cytotoxic T lymphocytes destroy infected cells directly. Memory cells remain for any antigen the body has met before, so when the same antigen returns, far more antibody is made far more quickly. In the diagram you can move from day 0 to day 70 and compare the primary immune response after antigen X first arrives with what happens on day 35, when antigen X returns together with a new antigen, Y.

Why We Stay Healthy Though Pathogens Keep Getting In

🛡️ Immunity, the Body's Defense System

①Huge numbers of bacteria and viruses enter the body every day, but most are removed before they cause disease

②The first line is nonspecific defense: skin and mucous membranes keep pathogens out, and phagocytes swallow those that get in, whatever their kind

③The next line is specific defense: lymphocytes recognize a particular antigen, attack only that antigen and leave memory cells behind

Primary and Secondary Immune Responses

Time (days)day 70

💡 What to Look for in the Graph

①When antigen X arrives for the first time, a lag of several days with almost no antibody is followed by a slow, small rise in antibody: the primary immune response

②During the primary response, memory cells that recognize antigen X are made, and they remain after the antibody level falls

③When antigen X returns on day 35, the memory cells quickly become plasma cells and antibody X rises sooner and much higher than in the primary response: the secondary immune response

④For antigen Y, which arrives for the first time that same day, there are no memory cells, so antibody Y rises slowly and little, as in a primary response. Each antibody binds only its own antigen

Types of Defense

Classifying immunity

Types of Defense

Immunity Comparison

Humoral and Cell-Mediated Immunity

Item	Humoral immunity	Cell-mediated immunity
Main cells	B lymphocytes → plasma cells	cytotoxic T lymphocytes
How it works	antibodies bind antigens	destroys infected cells directly
Main targets	pathogens and toxins in body fluids	virus-infected cells, cancer cells
Memory	memory B cells (make antibody fast in the secondary response)	memory T cells

Worked Examples

Example 1

Defense by skin, mucous membranes and phagocytes, and defense by antibodies made by B lymphocytes: what kind of defense is each?

A response that works the same way whatever the pathogen is nonspecific defense; one tailored to a particular antigen is specific defense.

Skin, mucous membranes and phagocytes act regardless of the kind of antigen, so they are nonspecific defense; antibody production by B lymphocytes is tailored to a particular antigen, so it is specific defense, namely humoral immunity.

▸ skin, mucous membranes and phagocytes: nonspecific defense; antibody production: specific defense (humoral immunity)

The test is whether the response changes with the kind of antigen. Antibodies bind only particular antigens, so any defense that involves antibodies is specific.

Example 2

Using immune memory, explain why a person who has been vaccinated rarely gets sick when the same pathogen enters later.

The vaccine's antigen sets off a primary immune response, which makes antibodies along with memory cells that recognize that antigen.

When the same pathogen enters, the memory cells quickly become plasma cells and make a large amount of antibody fast, so the pathogen is removed before it can multiply.

▸ the vaccine leaves memory cells, so a real infection meets a fast, strong secondary response

The antibody X curve after day 35 in the graph is exactly this situation. Memory cells from a vaccine respond only to their own antigen, so, like antigen Y in the graph, other pathogens get no head start.

Summary

The Path of Humoral Immunity

antigen enters → macrophage presents the antigen → helper T lymphocyte is activated → B lymphocytes become plasma cells and memory cells → antibodies are made

antibodies bind antigens, disarming the pathogen and making it easier for phagocytes to engulf

Secondary Immune Response

memory cells → same antigen returns → quickly become plasma cells → much more antibody

a vaccine works by introducing a killed or weakened antigen in advance so that memory cells are ready

CSAT-style problem, adapted

Which statement about human immunity is INCORRECT?

①Skin and mucous membranes are part of nonspecific defense.

②In humoral immunity, B lymphocytes produce antibodies.

③In cell-mediated immunity, T lymphocytes destroy infected cells directly.

④A single antibody binds many different kinds of antigen.

⑤The secondary response happens quickly because of memory cells.

▸ ④ A single antibody binds many different kinds of antigen.

Antibodies are specific: each kind of antibody binds only the particular antigen whose structure fits it.

So ④, which says one antibody binds many kinds of antigen, is incorrect.

🎯 Exam Points

①Nonspecific defense consists of skin, mucous membranes, phagocytosis and inflammation, and does not distinguish between kinds of pathogen

②Specific defense is tailored to a particular antigen and includes humoral immunity (antibodies) and cell-mediated immunity (cytotoxic T lymphocytes)

③Antibodies are specific: each antibody binds only the antigen that fits it

④Because of memory cells, the secondary response is faster than the primary and produces more antibody, which is how vaccines work

⑤ABO blood type is set by agglutinogens (antigens) on red blood cells and agglutinins (antibodies) in plasma, which decide whether clumping occurs in a transfusion

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Mendelian Genetics

Last updated 2026-10-17

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