What is immune System? Its Types, Mechanisms & Function

The immune system is the body’s defense against harmful agents. It provides immunity.

Immunity is the ability of the body to resist all the forms of organisms and their toxins which tend to damage the body cells and tissues.

This resistance to particular infectious microbes or toxins is done by the specific action of antibodies or sensitized white blood cells released by the immune system.

Structure of the immune system:

The immune system comprises of

  • Organ-based
  • Cellular-based

The organs of the immune system are distributed in different places of the body. These are differentiated as

Primary lymphoid organs: Thymus, Bone marrow

Secondary lymphoid organs: lymph nodes, Spleen, and mucous-associated lymphoid tissue (MALT)

The organs of the immune system


The cellular-based immune system comprises immune cells like monocytes, macrophages, lymphocytes, basophils, neutrophils, and eosinophils.

Cells of the immune system involved in body defense
Immune cells involved in body defense are the white blood cells.


The immune organs help in either the formation of immune cells or help to trap the pathogens or to expose them to immune cells.

These immune cells perform the actual function of defense either by producing antibodies or by eating the pathogens by phagocytosis.

Types of immunity

This immunity is of two types as

  1. Natural or innate immunity
  2. Acquired or adaptive immunity

Natural immunity is the general resistance to microbes, toxins, and other infections. This is not specifically directed against a particular organism or strain of microbes. This is also a part of physiology. This includes

a) Phagocytosis of microbes like protozoa, bacteria, fungi, etc. by the white blood cells.

b) Acidic and enzymatic destruction of foreign microbes in the digestive tract.

c) Prevention of invasion through the skin.

d) Destruction of foreign organisms and cells in the blood due to the action of lysozymes, polypeptides, and natural killer cells.

Thus natural immunity is the non-specific defense that keeps the body free from pathogenic microorganisms.

Acquired immunity

This form of immunity develops as a result of exposure of the body to external infectious microbes and toxins. It is powerful and extremely specific against individual microbes like pathogenic bacteria, protozoa, virus, etc. Here the body will have developed specific counter effects in the form of antibodies and killer cells, which can destroy the foreign agent if exposed again. This phenomenon of agent-specific immunity is used in mass vaccination to prevent future infectious attacks.

This acquire immunity works by two mechanisms as

a) Humeral immunity or B-cell immunity

b) Cell-mediated immunity or T-cell immunity.

Humeral immunity works by the release of antigens or immunoglobulins by B-cells. These antibodies bind to the pathogenic microbes and neutralize them.

Cell-mediated immunity works by direct destruction of foreign cells. This occurs due to the cells like the macrophages, neutrophils, and natural killer cells

Mechanism of immunity

In short, the body recognizes the foreign cells through their surface antigens. As a response, it produces antibodies and phagocytic cells to destroy those foreign cells.

For a substance to be antigenic, it should have a molecular weight of 8000or more. And an antigen is mostly a protein or large polysaccharide molecule. When it is introduced into the blood the stimulates antibody production.

An antibody is again a protein substance which binds to an antigen, there are five classes like Immunoglobin G (IgG), IgA, IgM, IgE, and IgD.

Type of antibody Its Character Functions and Effects
IgA This antibody is found in body secretions like the saliva and milk Prevents the antigens from passing through the epithelial membranes and deep tissues.
IgD It is present on the surface of B-lymphocytes They bind to antigens and activate the B-cells to release antibodies.
IgE This antibody is present on the cell membranes of Mast cells, basophils etc. This antibody activates inflammatory response. They are found in profuse numbers during allergies.
IgG This is the most largest and commonly found antibody. It attacks wide range of pathogens and even crosses the placenta to protect the fetus in the womb
IgM They are found in large numbers during primary acquired immunity. It activates the compliment system.

Antigen recognition

They possess cell surface receptors to several extracellular molecules receptor for cytokines, selectin, integrins, and FC (constant fragment of an antibody. These receptors recognize the organism and initiate intracellular mechanisms in macrophages. The antigen becomes recognizable can also get coated by antibodies or complement. This process is being termed opsonization.

Phagocytosis: Phagocytosis is defined as the process of engulfment of solid particulate material by the cells (cell-eating). The cells performing this function are called phagocytes. There are two main types of phagocytic cells.

  1. Polymorphonuclear neutrophils (pmns): which appears early in the acute inflammatory response, sometimes called macrophages.
  2. Circulating monocytes and fixed tissue mononuclear Phagocytes are commonly called macrophages.

Secretory function: macrophages secrete important biomolecule substance as follows.

  1. Cytokines (IL-1, IL-2, IL-6, IL-8, IL-10, IL-12, Tumour necrosis factor-α) and prostaglandins (PGE, Thromboxane-A, leukotrienes)which are chemical mediators of activates other leukocytes.
  2. Secretory protein involved in wound healing e, g. Collagenase, elastase, fibroblast growth factor, angiogenesis factor.
  • Acute phase reactants, e.g., fibronectin, microglobulin.

Antigen presentation: The macrophages are unable to lyse an antigen or an organism; the next best course adopted by them is to act as antigen-presenting cells for presenting to immunocompetent T cells(subtype CD4+ OR CD8+ cells) or to B cells.

Basophils and mast cells: Basophils are a type of circulating granulocytes(0-1%)while mast cells are their counterpart seen in tissues, especially in connective tissues around blood vessels and submucosallocationBasophiles and mast cells have IgE surface receptor thus on coming in contact with antigen binding to IgE(e.g., allergic reaction and parasites), these cells get activated and release granules, i.e., degranulate. These granules contain substances such as Histamine, platelet-activating factor, heparin, and certain chemical mediators.

Mast cells and basophils are involved in mediating inflammation in allergic reactions and have a role in wound healing.

Neutrophils: polymorphonuclear neutrophils (PMNs) are normally the most numerous of the circulating leucocytes (40-75%). The cytoplasm of the OMNs contains lysosomal granules of three types: primary, secondary, tertiary. The PMNs function as similar to macrophages and are therefore appropriately referred to as macrophages owing to their role as the first line of defense against an invading foreign organism in the body. The cells have limited sizes and types of organisms to be engulfed.

Eosinophils: The Eosinophils are also circulating granulocytes(1-6%). These cells play a role in allergic reactions and in intestinal helminthiasis. The granules of Eosinophils contain lysosomal enzymes, peroxidases, and chemical mediators of inflammation(e.g.prostaglandins, leukotrienes).

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