The Immune System Explained: Defenses, Cells, and Immunity Types

Posted on Jul 13, 2025 in Biomedical Sciences

The Human Immune System: Defenses and Responses

The immune system is responsible for the body’s defenses. Key organs include:

  • Bone marrow
  • Thymus
  • Spleen
  • Lymph nodes
  • Peyer’s patches
  • Tonsils
  • Adenoids

Immune Barriers: The Body’s First Lines of Defense

Immunity is broadly categorized into two main types:

1. Nonspecific (Natural) Immunity

This constitutes the primary and secondary barriers, providing immediate, general protection.

a) External Barriers

  • Physical: Skin (keratin), cilia in the bronchi.
  • Chemical: pH of the skin, stomach acid.
  • Biological: Autochthonous bacterial flora.

b) Internal Barriers (Inflammatory Response)

The inflammatory response is a crucial internal defense mechanism. It involves the formation and production of various substances. The arrival of a pathogen triggers the release of histamine, lymphokines, and the activation of the complement system. This also leads to the arrival of macrophages, granulocytes, and NK cells. The inflammatory response is characterized by:

  • Pain
  • Redness
  • Swelling
  • Heat

2. Specific (Acquired) Immunity

This constitutes the tertiary barrier, characterized by its memory and specificity. It is divided into two main types:

1. Humoral (Antibody-Mediated) Immunity

This type of immunity is carried out by B cells. When an antigen (Ag) binds to B lymphocytes, they differentiate into plasma cells and produce antibodies (Ac). These antibodies bind to the antigen. Memory cells remain in circulation, so if the same antigen appears again, they quickly transform into plasma cells, leading to a much faster response.

2. Cellular Immunity

This is primarily conducted by T cells. It is slower to respond to an antigen. Macrophages digest the antigen into peptide fragments and synthesize MHC (Major Histocompatibility Complex) molecules. MHC molecules recognize and bind to these peptides, transporting them to the cell membrane. T cells then recognize and bind to these MHC-peptide complexes. There are several types of T cells:

  • CD8 Cytotoxic T Cells: Destroy infected cells.
  • CD4 Helper T Cells: Release lymphokines that stimulate the production of CD8 cells.
  • Memory T Cells: Remain for years; if the same pathogen appears again, they quickly proliferate to prevent disease.
  • NK (Natural Killer) Cells: Large granular cells active against cancer cells.

Immune Responses: Primary and Secondary

Primary Immune Response

This is the response that occurs upon the first contact with an antigen.

Secondary Immune Response

This occurs upon subsequent (second or third) exposure to the same antigen. It is much faster and more robust.

Types of Immunity

1. Congenital Immunity

This type of immunity is inherited.

2. Acquired Immunity

Acquired immunity can be further categorized:

a) Natural Acquired Immunity

  • Passive: Acquired during infancy (e.g., maternal antibodies passed through the placenta or breast milk).
  • Active: Acquired after suffering a disease (e.g., developing antibodies after an infection).

b) Artificial Acquired Immunity

  • Passive: Administered via sera (antibodies are injected against specific pathogens; this is for immediate healing and does not produce memory cells).
  • Active: Via vaccines (killed or attenuated pathogenic agents are injected, which stimulates the body to produce memory cells and antibodies).

Key Immunological Components

Antigen (Ag)

Antigens are exogenous or endogenous substances of varied chemical nature. They induce the formation of antibodies (Ac) and react with them via specific sites called epitopes.

Antibody (Ab)

Antibodies are glycoproteins, also known as immunoglobulins (IgG, IgA, IgM, IgD, IgE). They are produced in response to an antigen. Antibodies are composed of four polypeptide chains: two heavy and two light, linked by disulfide bridges. Each antibody has two regions: a variable region (antigen-binding domain) and a constant region (effector domain). Antibodies are not present upon the arrival of an antigen; rather, lymphocytes are trained to proliferate and produce antibodies in response to an antigen. This process is called clonal selection.

Antigen-Antibody Reaction

An antigen binds to an antibody, forming an antigen-antibody complex. This complex can:

  • Agglutinate antigens.
  • Block the pathogen’s activity.
  • Stimulate the production of macrophages.
  • Activate the complement system, which targets the pathogen (opsonization).