The Immune System

The human defense against pathogens

The Immune System

The function of the immune system in the human body is to prevent infection and to rid the body of established infections. The defense mechanism provided by the immune system is known as immunity. Immunity refers to the bodies ability to defend itself against external threats, known as antigens. Antigens can be viruses, bacteria, cells, or other substances that could potentially cause the body harm. There are two general categories of immunity: innate immunity and adaptive immunity.

Innate immunity is the human body’s first line of defense. It provides initial protection from infection. Part of the innate immune system includes the skin, white blood cells, and complement factors. Innate immunity is generally referred to as non-specific immune responses.

Adaptive immunity is the more complex and “memory-oriented” type of immunity. Adaptive immunity utilizes cells, known as lymphocytes, to effectively fight infection against microbes that cannot be eliminated by the innate immune response. There are two types of adaptive immunity: humoral immunity and cell-mediated immunity. Humoral immunity provides defense from organisms outside the cell, while cell-mediated immunity provides defense from organisms inside the cell. Adaptive immunity is generally referred to as a specific immune response.

Cells of the Immune System

The participating cells in the immune system response are white blood cells, also known as leukocytes. All of the following cell types fall into this category.


The immune system contains three kinds of lymphocytes: B lymphocytes, T lymphocytes, and natural killer cells. B lymphocytes secrete special proteins known as antibodies. B lymphocytes are the cells responsible for the mediation of humoral immunity. B lymphocytes are created from stem cells in the bone marrow, where they also mature.

T lymphocytes recognize antigens on the surface of antigen-presenting cells. Like B lymphocytes, T lymphocytes are created from stem cells in the bone marrow, except they mature in the thymus. B lymphocytes and T lymphocytes are the primary players in adaptive immunity. On the other hand, natural killer cells are responsible for attacking microbes as a part of the innate immune response.

T lymphocytes are differentiated by molecules on the cell surface known as CD molecules (cluster of differentiation). The kinds of T lymphocytes can be identified based on the type of CD molecule expressed on the cell surface. You will most likely encounted CD4+ T cells and CD8+ T cells. CD4+ cells are T lymphocytes that include T helper cells and T regulatory cells. CD8+ cells are T lymphocytes that include cytotoxic T cells (CTLs).

Antigen-Presenting Cells

Antigen-presenting cells are immune system cells that are located in the skin, mucous membranes, and respiratory tract. These cells capture antigens and “present” the antigens to lymphocytes located in the lymphoid tissues (hence, the name “antigen-presenting). An important antigen-presenting cell is known as a dendritic cell.

Effector Cells

Effector cells are cells that eliminate microbes. These cells may be specialized B lymphocytes or T lymphocytes, macrophages, or granulocytes. Types of granulocytes include neutrophils, basophils, and eosinophils.

Tissues of the Immune System

Tissues of the immune system include lymphoid tissue and lymphatics. These tissues are the location that T lymphocytes and B lymphocytes begin to mature and become able to respond to antigens. Immune system tissues include the thymus, the spleen, and lymph nodes.


The thymus is an immune system organ located in the thoracic cavity (chest). Unlike most organs, the thymus is larger in children than it is in adults.


Lymph Nodes

Get started now.

Sign up