The Human Immune System

Learning Objectives

Distinguish between innate and adaptive immunity.

Describe the major components of the innate immune system.

Explain the roles of B cells and T cells in adaptive immunity.

Describe the function of antibodies and memory cells.

The Body's Defense Force

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders (pathogens) like bacteria, viruses, and fungi. It has two main branches.

1. Innate (Nonspecific) Immunity

This is the body's first line of defense. It is present from birth and acts rapidly against a broad range of pathogens.

Physical Barriers: Skin, mucous membranes.

Chemical Barriers: Stomach acid, enzymes in saliva.

Cellular Defenses:

Phagocytes (e.g., macrophages): Cells that engulf and digest pathogens.

Inflammation: A localized response to injury or infection characterized by redness, heat, swelling, and pain. It helps to contain the infection and recruit immune cells to the site.

2. Adaptive (Specific) Immunity

This is a more specialized defense system that develops over time in response to exposure to specific pathogens. It is characterized by specificity and memory. There are two main arms:

A. Humoral Immunity (B Cell-mediated)

This branch targets pathogens floating in body fluids ('humors').

B cells (B lymphocytes) that recognize a specific antigen are activated.

Activated B cells differentiate into plasma cells, which are antibody factories.

Antibodies are Y-shaped proteins that bind to specific antigens, marking them for destruction by phagocytes or neutralizing them directly.

B. Cell-Mediated Immunity (T Cell-mediated)

This branch targets the body's own cells that have become infected or cancerous.

Helper T cells are the 'generals' of the adaptive immune system. They become activated and then help to activate B cells and other T cells.

Cytotoxic T cells (or 'killer' T cells) directly recognize and kill infected body cells by releasing toxic chemicals.

Immunological Memory

After a primary infection is cleared, some activated B and T cells become long-lived memory cells. If the same pathogen enters the body again, these memory cells allow for a much faster and stronger secondary immune response, often preventing illness. This is the principle behind vaccination.

Key Terms

Innate Immunity: The non-specific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body.
Adaptive Immunity: The antigen-specific immune response that creates an army of immune cells specifically designed to attack that antigen, and also creates memory cells to protect against future encounters.
Antibody: A blood protein produced in response to and counteracting a specific antigen.
B Cell: A type of lymphocyte, developed in the bone marrow, that produces antibodies.
T Cell: A type of lymphocyte, developed in the thymus gland, that plays a central role in cell-mediated immunity.